Support thinning and renaming modules

GHC supports module thinning and module renaming. See https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/packages.html#thinning-and-renaming-modules. This feature is older than Backpack, but AFAICT was only exposed in Cabal as part of the Backpack work when they added the mixins field. We could implement this with an API like the following:

haskell_library(
  name = "foo",
  deps = [":bar", ":baz"],
  imports = {":bar": "Data.Bar1 as Data.Bar1_2, Data.Bar2 as Data.Bar2_2"},
)

We are limited to just strings as dictionary values by Skylark's current API. Which it would be annoying to have to parse. But fortunately we don't have to, and can instead pass as-is to GHC. We propose imports as the attribute name by symmetry with exports proposed in #357.

Start with the proposed imports attribute on haskell_library and read the linked GHC documentation on thinning and renaming modules. Review the related exports proposal in #357, then define the rule and integration points needed for string-valued module mappings; done means the example API works and GHC receives the mappings unchanged.