Module reexports, fixing #8407.
The general approach is to add a new field to the package database, reexported-modules, which considered by the module finder as possible module declarations. Unlike declaring stub module files, multiple reexports of the same physical package at the same name do not result in an ambiguous import. Has submodule updates for Cabal and haddock. NB: When a reexport renames a module, that renaming is *not* accessible from inside the package. This is not so much a deliberate design choice as for implementation expediency (reexport resolution happens only when a package is in the package database.) TODO: Error handling when there are duplicate reexports/etc is not very well tested. Signed-off-by: Edward Z. Yang <firstname.lastname@example.org> Conflicts: compiler/main/HscTypes.lhs testsuite/.gitignore utils/haddock