Track language extensions and language flavours in the solver.

Every package now "depends" on all language extensions
(default-extensions and other-extensions) and language flavours
(default-language and other-languages) it declares in its cabal file.

During solving, we verify that the compiler we use actually
supports selected extensions and languages. This has to be done
during solving, because flag choices can influence the declared
extensions and languages being used.

There currently is no equivalent check performed on the generated
install plans. In general, cabal-install performs a sanity check
on the solver output, checking that the solver e.g. indeed includes
all the declared dependencies of a package. There is no such
double-checking for language extensions. This is not really
problematic, as all that this change does is to make the solver
more conservative rather than less. However, having a sanity check
available might ultimately be nice to have.