Previously, we stated that registration was needed if any build target
was a library.  Actually, this is not enough: if we have a build target
on an executable which in turn depends on the library, we must register
it.

To actually get this information, I had to refactor ComponentsGraph to
return the actual *graph*, so I could do a reverse closure on it, giving
me the set of components that actually depend on the library.  That info
then gets plumbed through ElaboratedPackage and then used to determine
if registration is necessary.

Fixes #4450, but it's possible there is another variant of the bug
that occurs if the executable to be built does NOT depend on the
library.

I also needed to add a new hasNewBuildCompatBootCabal helper to the
test suite to check if we can actually build the Custom setup
with the boot Cabal.

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