Watch arbitrary file in repo rules

* Starlark API changes
  * New method `rctx.watch()` that watches a path. As of right now, said path must exist and be a regular file, but that will change in follow-ups.
  * Existing method `rctx.read()` gets a new optional string parameter `watch` that defaults to `auto`. This causes the file read to be watched as well by default, even if it's not addressed by a label. See `rctx.read()` docs for the semantics of `watch`.
  * Both changes apply to `mctx` as well, except that no files outside the current Bazel workspace may be watched for `mctx`.
* Marker file format changes
  * `FILE:` marker file entries can now be `FILE:` followed by either an absolute path (starts with a singular '/'), or a label-like ... thing (which is a label but has a slash in place of a colon). This is because we might be watching something inside a repo that is _not_ under a package.
  * Same change will happen to the `accumulatedFileDigests` sections of the module lockfile.
* Code changes
  * `RepoRecordedInput.File` subclasses are renamed to `FileOutsideWorkspace` (absolute paths) and `FileInsideWorkspace` (label-like things) instead to better reflect their meaning.
  * Unified the file watching/digest checking logic in SingleExtensionEvalFunction with the RepoRecordedInput stuff. Follow-ups will unify more.
  * Moved RepoRecordedInput to its own BUILD target as it's being used outside the repo fetching machinery (namely, module extensions)
  * Since `FileOutsideWorkspace` has to be an absolute path (never relative), we don't need a "base directory" for the RepoRecordedInput parser. What we really need is just the FileSystem object, which we can get from the PathPackageLocator available in Skyframe. Refactored code to reflect that.

Work towards https://github.com/bazelbuild/bazel/issues/20952.

Closes #21230.

RELNOTES: Added a new method, `repository_ctx.watch()`, which asks Bazel to watch for changes to an arbitrary file. When said file changes, Bazel will refetch the repo.
PiperOrigin-RevId: 607097422
Change-Id: Ib96a49461deddd7f4c786bd1c227de18b2dd71d7
29 files changed
tree: 85d642b16a4700fa0f61ae654b2dcb35f99ec5a2
  1. .bazelci/
  2. .github/
  3. examples/
  4. scripts/
  5. site/
  6. src/
  7. third_party/
  8. tools/
  9. .bazelrc
  10. .bazelversion
  11. .gitattributes
  12. .gitignore
  13. AUTHORS
  14. bazel_downloader.cfg
  15. BUILD
  16. CHANGELOG.md
  17. CODE_OF_CONDUCT.md
  18. CODEOWNERS
  19. combine_distfiles.py
  20. combine_distfiles_to_tar.sh
  21. compile.sh
  22. CONTRIBUTING.md
  23. CONTRIBUTORS
  24. distdir.bzl
  25. extensions.bzl
  26. LICENSE
  27. maven_install.json
  28. MODULE.bazel
  29. MODULE.bazel.lock
  30. rbe_extension.bzl
  31. README.md
  32. repositories.bzl
  33. requirements.txt
  34. SECURITY.md
  35. WORKSPACE
  36. WORKSPACE.bzlmod
  37. workspace_deps.bzl
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