Add support for convenience libraries

[rl]

I'd like to have a package with several binaries which all use a convenience library included in that same package. The convenience library should NOT be installed. For this to work, Cabal needs two things. Firstly, it must allow dependencies within a package such that I could describe the library and have the binaries depend on it. Secondly, it should have an "installable" flag, similar to "buildable". A thing that's not installable would be built but not installed. The same mechanism could be used for package-internal tools.

[duncan]

There is the constraint that there be at most one installable library so I'm not sure a bool flag is quite right.

[duncan]

This would mean that the lib would always have to be built statically even if the user requests dynamic libs.

So this feature mainly depends on the ability to use build-depends: on a lib defined within the same package (whether that lib is to be installed or not). Similarly for build-tools.

This would also solve ticket #89.