feat: add option to persist rule index

[jbedard]

What type of PR is this? Feature

What package or component does this PR mostly affect? cmd/gazelle

What does this PR do? Why is it needed?

Add the ability to persist the RuleIndex across runs to allow updating only the specified directories when indexing is required.

Which issues(s) does this PR fix?

Ref: #1181

[jbedard]

Just to re-iterate a few points and limitations of this:

This is EXPERIMENTAL with no api guarantees. 99% chance there will be breaking changes. You should never commit this file to source-control.

When this EXPERIMENTAL feature is used:

• the resolver.Imports/Embeds() calls for the entire workspace (the "rule index") is written to disk
• when specifying dirs (default: .) on the command line only those directories are updated in the "rule index"
• only the dirs specified on the command line will be traversed, therefore:
  • only those dirs will have Imports/Embeds() invoked
  • only those dirs will have Fix(), GenerateRules() invoked
  • only those dirs will have BUILDs updated, warnings generated etc.
  • only those dirs will have things like resolve warnings outputted

If resolver.Imports/Embeds() have side-effects then this won't work very well when they are cached and not invoked. Don't do that.

There is no cache invalidation here. It is entirely up to the user to pass the correct list of dirs that require reprocessing.

FUTURE: if those updated dirs changed what rules they generate and the Imports/Embeds() of those rules change it is possible that other rules outside dirs are now referencing something incorrect. I think if Imports/Embeds() of those dirs change we might want to re-run Fix + GenerateRules for all directories.