Convenience libraries can have dependencies declared using
build-depends.  Unfortunately, these dependencies are special:
in particular, the version range is meaningless because they
always refer to the same package they were specified in.

So, this infelicity means that the solver has to explicitly
filter out these internal dependencies.  I also had to fix
validation to ignore these internal dependencies as well.

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