feat(lint): plumb exit codes as outputs rather than dropping them

[alexeagle]

This allows downstream tools to return a correct exit code based on 'errors were reported by at least one linter'

Also fixes our bug where "empty output" is assumed to mean success, which was forcing us to stay on an older version of ruff.

Note that lint.sh doesn't try to read these files and exit with the right code. I don't really care about that right now, but will file an issue to come back to it.

Work towards #86

---

Changes are visible to end-users: yes

It's only slightly visible, in that lint.sh now prints exit codes as well:

rules_lint/example$ ./lint.sh src:unused_import
INFO: Analyzed target //src:unused_import (5 packages loaded, 492 targets configured).
INFO: Found 1 target...
INFO: Elapsed time: 0.309s, Critical Path: 0.00s
INFO: 1 process: 1 internal.
INFO: Build completed successfully, 1 total action
INFO: Build Event Protocol files produced successfully.
From bazel-out/k8-fastbuild/bin/src/ruff.unused_import.aspect_rules_lint.report:
src/unused_import.py:13:8: F401 [*] `os` imported but unused
Found 1 error.
[*] 1 fixable with the `--fix` option.

From bazel-out/k8-fastbuild/bin/src/ruff.unused_import.aspect_rules_lint.exit_code:
1

From bazel-out/k8-fastbuild/bin/src/flake8.unused_import.aspect_rules_lint.report:
src/unused_import.py:13:1: F401 'os' imported but unused

From bazel-out/k8-fastbuild/bin/src/flake8.unused_import.aspect_rules_lint.exit_code:
1

• Searched for relevant documentation and updated as needed: yes
• Breaking change (forces users to change their own code or config): no
• Suggested release notes appear below: no

Test plan

• Covered by existing test cases

[gregmagolan]

🧙