Clean up and polish Bazel's external dependencies declarations:

 - Gathered all repository declarations in `repositories.bzl` file.
 - Removed most of repo definitions in `distdir_deps.bzl` and renamed it to `workspace_deps.bzl`, the rest repo definitions are only used in WORKSPACE suffix and test setup.
 - Added a repo cache for the workspace repos so that we can decouple repo cache used by
     - dependencies used to build and test Bazel: cached by `//src:test_repos`
     - dependencies embedded in MODULE.tools: cached by `@bazel_tools_repo_cache//:files`
     - dependencies embedded in WORKSPACE suffix: cached by `@workspace_repo_cache//:files`
 - Removed unused macros from `distdir.bzl`

This PR largely simplifies `distdir_deps.bzl`, which is also loaded in internal codebase because `gen_workspace_stanza` is a build rule.

Closes #20042.

PiperOrigin-RevId: 580212706
Change-Id: I91a9f7bbf89b9af15fdb98f387d95a40a89e7700