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

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

In #4017, hvr reported that when he used --allow-older/--allow-newer,
there was an assert failure in toConfiguredComponent.  Indeed
the problem was that toConfiguredComponent was testing version
ranges of build-depends to determine which package to select, but
there was no satisfying one (since the build-depends field had
not been updated.)

After thinking about this for a bit, it seemed a bit bogus for
us to be doing another version check at this late phase; we
already picked dependencies earlier in the configuration process.
So I decided to drop it.

To drop it, however, I needed to remove support for a feature (discussed
in #4020), which uses version ranges to disambiguate whether or not a
dependency is on an external package or an internal package.  This
feature doesn't seem to be very useful.  If someone asks, I'll
check on Hackage to see if anyone is using it.

Also added some useful extra debug info.

Fixes #4020 and #4017

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

When a library/executable is unbuildable, the solver may still
return a reference to it.  Handle this gracefully in ProjectPlanning
with a nice error message, rather than an assert failure.

Fixes #3978.

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