feat(toolchain): support specifying x.y versions in transitions

[aignas]

This is inspired by how rules_go is registering their toolchains.

Their toolchains have multiple target_settings values. This
allows for a simpler passing of X.Y version to the py_binary and
py_test rules and does not strictly require us to provide the APIs
that pass the full python version value as the closure. This is only
possible because #1555 introduced working aliases and now we can also
have this.

Summary:

• refactor: move the toolchain_def to starlark as opposed to templating
• refactor: move the version setting as well
• feat: support matching on X.Y versions
• feat: X.Y.Z will match if X.Y is used as python_version flag and the
  MINOR_MAPPING has "X.Y": "X.Y.Z".
• test: add tests checking the generated config settings.
• doc: add an example of how we could use the transition files directly

See https://github.com/bazelbuild/rules_go/blob/master/go/private/go_toolchain.bzl#L181

[aignas]

Hah, the build is unhappy, but not in all situations, however, wanted to submit this as it was in my local branch and #1555 added the missing pieces that I needed to get this almost working.

[rickeylev]

LGTM. Just either remove the optional MINOR_MAPPING check, or put a comment about the case it is serving.

[aignas]

I'm gonna merge this, but if you have a comment on how to make the test for the generated code better, let me know. It seemed like visual inspection was the easiest way for now.

[rickeylev]

For testing, analysis_test can be used with selects and a small custom rule to hold values.

subject = rule(
  implementation = lambda ctx: None,
  attrs = {"match_a": attr.bool()}
)

def _test_config_matching(name):
  subject(
    name="subject",
    match_a = select({"is_python_3.4.5": True}, "//conditions:default": False})
  )
  analysis_test(
    ...,
    config_settings = {
      "@rules_python//python/config_settings:python_version": "3.4"
    }
  )

def _test_config_matching_impl(env, target):
  env.expect.that_target(target).attr("match_a", factory=subjects.bool).equals(True)

Oh, actually, it looks like there are some tests that do basically this in tests/config_settings/construct_config_settings_test.