Implement Clippy Aspect & Build Rule

[smklein]

Follow-up from #338

Working on issue: #304

[smklein]

As a final validity test, I ran:

cd examples
bazel build --aspects=@io_bazel_rules_rust//rust:rust.bzl%rust_clippy_aspect --output_groups=clippy_checks //...

This uncovered a handful of issues -- which have since been fixed. Of note:

• Many of the "compilation-command-building" arguments assume they ingest ctx, and operate on ctx.files or ctx.file directly. When building directly on a target, this is fine, but in the context of an aspect, this is non-ideal, since the actual desired file/files exist within ctx.rule.files or ctx.rule.file, respectively. The functions have been refactored to act on a user-supplied file/files in these cases.
• Rather than trying to "recreate" CrateInfo, I realized I could just access it directly from the Aspect's target, so I'm doing that now.

@damienmg I appreciate the help getting this patch reviewed, but I don't want to slide something in after the "+2" -- would you be willing to take another look?

Also, I'm currently running clippy on the examples/ directory manually - I think it would be cool if we could actually add that to the automated build. I tried adding a top-level rust_clippy targets that includes all rust_(library,binary,test) targets, but this would require significant changes to the visibility within examples/.

Any advice on this? How could I go about changing the buildkite script to run this as a check on future PRs?

[damienmg]

So for test, I think this is fine as it is.

You could also modify the preubmit.yml file to add the test you want but I am fine with checking this PR in as-is.

[smklein]

So for test, I think this is fine as it is.

You could also modify the preubmit.yml file to add the test you want but I am fine with checking this PR in as-is.

Oh rad, didn't see that file! I added clippy_examples and clippy_failure as tests to the presubmit.