Most TODOs related to the new InstalledPackageId stuff.

We have to set the installedPackageId if it's missing.
There's no need for this to depend on the version of ghc which
is nice since this module is not supposed to be ghc specific.

The old one was always too generic and it now contrasts
better with the new installedPackageId field.

When registering, choose the InstalledPackageId.

 - When registering inplace, use "foo-1.0-inplace"

 - If this isn't GHC, just use "foo-1.0-installed"

 - When installing a package with GHC, call
   Distribution.Simple.GHC.libAbiHash to get the hash, and
   use "foo-1.0-<hash>".

Follow changes in Distribution.Simple.LocalBuildInfo (installedPkgs is
now an InstalledPackageIndex).

Distribution.Simple.Configure: follow changes to PackageIndex and
INstalledPackageInfo.

Add libAbiHash, which extracts a String representing a hash of
the ABI of a built library.  It can fail if the library has not
yet been built.

Follow changes to Distribution.Simple.LocalBuildInfo.componentPackageDeps.

Distribution.Simple.LocalBuildInfo: 

  - the LocalBuildInfo record contains an index of the installed
    packages; this has changed from PackageIndex InstalledPackageInfo
    to InstalledPackageIndex.

  - ComponentLocalBuildInfo stores the "external package dependencies"
    of the component, which was 
      componentPackageDeps :: [PackageId]
    and is now
      componentInstalledPackageDeps :: [InstalledPackageId]

  - we now export
      componentPackageDeps :: LocalBuildInfo -> [PackageId]
    (since to get the PackageId for an InstalledPackageId, you need
    to look it up in the InstalledPackageIndex, which is in the
    LocalBuildInfo)

  - similarly, previously
      externalPackageDeps :: LocalBuildInfo -> [PackageId]
    is now
      externalPackageDeps :: LocalBuildInfo -> [InstalledPackageId]

This part adds the InstalledPackageIndex type to
Distribution.Simple.PackageIndex.  Now that packages have a unique
identifier within a package database, it makes sense to use this as
the key for looking up installed packages, so InstalledPackageIndex is
a mapping from InstalledPackageId to InstalledPackageInfo.

Distribution.Simple.PackageIndex still supports other kinds of package
mappings: PackageIndex is a mapping from PackageName.

All the functions in the section "Special Queries" now work on
InstalledPackageIndex rather than PackageFixedDeps pkg => PackageIndex
pkg:

  topologicalOrder,
  reverseTopologicalOrder,
  dependencyInconsistencies,
  dependencyCycles,
  brokenPackages,
  dependencyClosure,
  reverseDependencyClosure
  dependencyGraph

Note: this patch doesn't build on its own, you also need the rest of
the patch series.

Compatibility with older GHCs.  When reading a package database
created by an older version of GHC without installedPackageIds, we
have to fake an InstalledPackageId for the internal
InstalledPackageInfo.

So, when reading in an InstalledPackageInfo from an older GHC, we set
the installedPackageId to be the textual representation of the
PackageIdentifier: i.e. <package>-<version>.  Now, previously the
depends field of InstalledPackageInfo was [PackageIdentifier], and is
now [InstalledPackageId], so we will read each PackageIdentifier as an
InstalledPackageId (a String).  The dependencies will still point to
the correct package, however, because we have chosen the
installedPackageId to be the textual representation of the
PackageIdentifier.

  
  NOTE: the patch has been split into 9 pieces for easy
  reviewing, the individual pieces won't build on their own.
  
Part 1 does the following:

Distribution.Package: 
  - define the InstalledPackageId type as a newtype of String

Distribution.InstalledPackageInfo:
  - add an installedPackageId field to InstalledPackageInfo
  - change the type of the depends field from [PackageIdentifier]
    to [InstalledPackageId]


The idea behind this change is to add a way to uniquely identify
installed packages, letting us decouple the identity of an installed
package instance from its package name and version.  The benefits of
this are

  - We get to detect when a package is broken because its
    dependencies have been recompiled, or because it is being
    used with a different package than it was compiled against.

  - We have the possibility of having multiple instances of a
    given <package>-<version> installed at the same time.  In the
    future this might be used for "ways".  It might also be
    useful during the process of upgrading/recompiling packages.

The key point is that we do not complain for packages
that do not depend on base at all (like ghc-prim).

Though I'm not at all sure I like the _ <- bind idiom.

We capture (and discard) the program output because we do not
want it printed to the console. We do not currently have a
specific variant for redirecting the output to /dev/null so
we simply use the variant that captures the output.

rolling back:

Fri Jul 10 22:04:45 BST 2009  Ian Lynagh <igloo@earth.li>
  * Don't ask for the output of running ld, as we ignore it anyway

    M ./Distribution/Simple/GHC.hs -2 +2
    M ./Distribution/Simple/LHC.hs -2 +2
Fri Jul 10 22:08:02 BST 2009  Ian Lynagh <igloo@earth.li>
  * Don't use the Stdout variant of rawSystemProgramConf to call gcc
  We ignore the output anyway

    M ./Distribution/Simple/Configure.hs -2 +3

We ignore the output anyway

Take a set of templates rather than file paths.

This does the mutual substituition of the installation
directory templates into each other.

The --verbose flag is now called --verbosity

Existing Setup scripts use it so we can't change it. Fixes #563.

the layout changed in the new GHC build system

rather than low-level System.Posix.Internals operations which are
about to go away.