Deduplicate module map flags in Swift compilation
Deduplicate module map flags in Swift compilation

Module map files being propagated through different providers seem to
ended up being treated like different modules, even though they point to
the same .modulemap files. Swift compilation in targets with a lot of
objc_library dependencies were getting a lot of duplicate flags. This
fixes that by deduplicate the module map list before passing them to the
compiler.

bazelbuild/rules_swift,bazelbuild/rules_swift