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 <ezyang@cs.stanford.edu>