[linter] Clarify status and use of rules.json and clean up generation

- Adds a doc at `pkg/linter/tool/machine/rules.json` with info about the file.
- Drops the disabled test that checked that the file is up to date.
- Drops inclusion of rule sets from the `rules.json` file.
- Consolidate the remaining logic for determining rule sets


Closes https://github.com/dart-lang/linter/issues/4756
Closes https://github.com/dart-lang/sdk/issues/55977
Contributes to https://github.com/dart-lang/sdk/issues/56835

Change-Id: Ic030640571c0f8cf553a218a2e32c5a767535564
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/388481
Auto-Submit: Parker Lougheed <parlough@gmail.com>
Reviewed-by: Devon Carew <devoncarew@google.com>
Commit-Queue: Devon Carew <devoncarew@google.com>
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>
Reviewed-by: Phil Quitslund <pquitslund@google.com>
Reviewed-by: Samuel Rawlins <srawlins@google.com>

PRESUBMIT.py

@@ -430,8 +430,6 @@ def _CheckAnalyzerFiles(input_api, output_api):
     #   content, when `pkg/analyzer/messages.yaml` is modified.
     # * Verify that `diagnostics/generate.dart` does not produce different
     #   content, when `pkg/analyzer/messages.yaml` is modified.
-    # * Verify that `machine.json` is not outdated, when any
-    #   `pkg/linter/lib/src/rules` file is modified.
     # * Maybe "verify_no_solo" for individual modified (not deleted test files
     #   in Analyzer-team-owned directories.