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 <ezyang@cs.stanford.edu>

Conflicts:
	compiler/main/HscTypes.lhs
	testsuite/.gitignore
	utils/haddock