Replace @scalafmt_default with toolchains target

[mbland]

Description

Removes the scalafmt_config() macro and replaces it with the new @rules_scala_toolchains//scalafmt:config target. Part of #1482.

Motivation

The new test/shell/test_dependency_versions.sh test found a problem with the previous implementation. The dev_deps extension in MODULE.bazel generated @scalafmt_default, leaving it invisible to rules_scala when it's not the main module:

ERROR: no such package
  '@@[unknown repo 'scalafmt_default' requested from @@rules_scala~]//':
  The repository
  '@@[unknown repo 'scalafmt_default' requested from @@rules_scala~]'
  could not be resolved:
  No repository visible as '@scalafmt_default'
  from repository '@@rules_scala~'

ERROR: .../tmp/test_dependency_versions/BUILD:52:20:
  every rule of type scalafmt_scala_test
  implicitly depends upon the target
  '@@[unknown repo 'scalafmt_default' requested from @@rules_scala~]//:config',
  but this target could not be found because of:
  no such package
  '@@[unknown repo 'scalafmt_default' requested from @@rules_scala~]//':
  The repository '@@[unknown repo 'scalafmt_default' requested from @@rules_scala~]'
  could not be resolved:
  No repository visible as '@scalafmt_default' from repository '@@rules_scala~'

Documentation for implicit attribute config of rules of type scalafmt_scala_test:
The Scalafmt configuration file.

ERROR: Analysis of target '//:ScalafmtTest' failed;
  build aborted: Analysis failed

The scalafmt_default_config() macro is already gone, and only scala_toolchains() invoked scalafmt_config(), making this a straightforward change.

[simuons]

Could we get rid of this flag? I mean scala_deps.scalafmt(default_config = "//path/to/my/custom:scalafmt.conf")
implies scalafmt = True,
For workspace sure we would need to specify boolean flag and pass config as there are no other way to render this in single repository.

[mbland]

Ah, that was just a mistake in the docs. Fixed the commit while rebasing on master, and the build is passing.

---

But speaking of the boolean flag in WORKSPACE, well... The idea from #1722 (comment) of using the module extension to instantiate user defined toolchains is haunting me. I started experimenting with the idea in mbland/bzlmod-toolchains-extension.

In the process, I found a way to make the scalafmt, scala_proto, and twitter_scrooge parameters accept either a boolean (where True implies "use all default settings") or a dict (which implies True). I also updated the semantics of scala/extensions/deps.bzl somewhat, hopefully making them clearer and more consistent. I also changed scala_proto(options) to scala_proto(default_gen_deps) to match setup_scala_proto_toolchains().

I don't know that I'm close yet to making it work for all user defined toolchains, but it should make it easier for users to configure the builtin toolchains more precisely. I'll send a pull request from that new branch soon after this pull request lands.