We need these types for their Read instances so that we
can still read older GHCs package db files when we make
changes to the current InstalledPackageInfo type, or the
types contained in it, like PackageIdentifier or License.

Actually it turns out that we don't need case insensitivity in many
cases, mosty just for simple lookups in the UI. For everything else
the ordinary Ord instance is much simpler. The fact that listing the
contents of a PackageIndex doesn't come out in Ord order actually
causes real problems in cabal-install and necessitates re-sorting.
So we should move to using PackageSet in most cases and just leave
the search and lookup operations in PackageIndex.

Now that we have removed the hsSourceDirs = [currentDir] default
from emptyBuildInfo it is now equal to nullBuildInfo.

  build-depends: foo ~1.2.*
means:
  build-depends: foo >=1.2 && <1.3
It's also valid everywhere else version ranges are used.

We changed the parsing of list fields in the .cabal file so that it
adds to the current value rather than replacing it. This allows you
to put multiple entries for a list field and they all get
concatenated. However that means that the '.' in the hsSourceDirs of
emptyBuildInfo is always added to and not replaced like we did
previously. That's not what we want in this case. We want to use '.'
for hsSourceDirs *only* if hsSourceDirs is otherwise null. As it
happens, due to the way the configurations code works, we're already
filling in the default if it'd otherwise be null so we do not need
'.' in the emptyBuildInfo at all.

It means we catch any fields that get added. As it happens we were
missing a field, though its value is supposed to be just [] which is
the same value as we got from the default emptyInstalledPackageInfo.

withUTF8FileContents now closes the file, so we need to force what we're
computing from the contents before it's gone.

Having it there confuses people.
They think they have to run it as part of the install process.

This allows libs that have only private modules or C code. This
might be used to make libs that have non-exposed modules and only
export C APIs. It could also be used to make packages that consist
only of C code. That might be useful for bindings where it may
make sense to split the C and Haskell code into separate packages.

When specified without any file name it is supposed to use
a default file name rather than be ignored completely.

Fixes ticket #187 finally (I hope).

We had a bug on windows where we open and read ghc's package.conf
database but because we did not consume the final newline we did
not close the file. Then when we called ghc-pkg to register a
package it failed because it could not rename the open file.

Safe block scoped reading of files.
These guarantee that the file gets closed.

Now look in the right place to find the pre-processed source
files belongign to executables. Fixes ticket #252.

Fixes ticket #249

This is actually Andrea Rossato's patch but it didn't merge
cleanly due to more recent changes. Fixes ticket #250.