This commit removes the 'ExTest' constructor from 'ExampleDependency'.
Test dependencies are now represented using the same
'ExampleDependency' constructors as library dependencies.  The only
difference between dependencies of different components is that they
are placed under different keys in 'D.C.ComponentDeps.ComponentDeps'.

Convenience libraries are package-private libraries
that can be used as part of executables, libraries, etc
without being exposed to the external world.  Private
libraries are signified using the

    library foo

stanza.  Within a Cabal package, the name convenience library
shadows the conventional meaning of package name in
build-depends, so that references to "foo" do not indicate
foo in Hackage, but the convenience library defined in the
same package. (So, don't shadow Hackage packages!)

This commit implements convenience libraries such that they
ARE installed the package database (this prevents us from
having to special case dynamically linked executables);
in GHC 7.10 and later they are installed under the same
package name as the package that contained them, but have
a distinct "component ID" (one pay off of making the distinction
between component IDs and installed package IDs.)

There is a "default" library which is identified by the fact
that its library name coincides with the package name.  There
are some new convenience functions to permit referencing this.

There are a few latent bugs in this commit which are fixed
in later commits in this patchset.  (Those bugfixes required
a bit of refactoring, so it's clearer if they're not
with this patch.)

Signed-off-by: Edward Z. Yang <ezyang@cs.stanford.edu>