Undo new-build support for convenience libraries.
The previous approach I took, though correct, was quite confusing. If I refactor InstallPlan to operate on a per-component basis, then we'll automatically get support for convenience libraries, which will ultimately cleaner. (While we won't be able to get rid of support for whole package installs, it will be safe to assume packages using convenience libraries also support one-shot configure.) I didn't revert the support in cabal install; I'm not planning on componentizing it. Signed-off-by: Edward Z. Yang <firstname.lastname@example.org>