Rename haskell_import to haskell_toolchain_library

[mboes]

This is a substantial breaking change. But a simple one to handle: it
can be done with

sed -i s/^haskell_import/haskell_toolchain_library/ **/BUILD{,.bazel}

everywhere. I considered marking this as deprecated instead, but that
won't help much: the deprecation spam will make things unbearable
without switching to the new name anyways.

The motivation for this change can be found in #440 and #838.
haskell_import was always poorly named, because it did not
correspond to precedent in the CC and Java rules. Whereas those rules
"import" concrete artifacts with some metadata into the dependency
graph, our old haskell_import was merely giving a name in the
dependency graph to stuff that is magically available from somewhere,
namely the GHC boot libraries.

I will be submitting a followup PR to implement a new haskell_import
that is similar in spirit to cc_import and java_import, once
I manage to solve/workaround the two following bugs:

• https://groups.google.com/forum/?nomobile=true#!topic/bazel-discuss/gQRd7CGmVu8
• Adding dynamic_library to create_library_to_link() causes NPE bazelbuild/bazel#8129

Meanwhile this PR just prepares the ground, by moving the existing
haskell_import out of the way and renaming it to
haskell_toolchain_library.

[aherrmann]

LGTM, I agree for the reasoning about rename vs. deprecate.

This should probably be listed in the changelog.

To clarify, how does this relate to haskell_import_new? It sounds like haskell_import_new could then be replaced by/extended to the new haskell_import as described in this PR's message.

[mboes]

Yes. TBH I don't quite understand haskell_import_new. Maybe @regnat can comment on whether this can replace haskell_import_new, which AFAIK was an experiment that at this point is only used in the rules_haskell test suite. haskell_import_new enables an alternative modular way to interoperate with Nix. I don't know whether we'll be able to recover that (though it's not very usable at the moment because it's very slow).