Add TagLib classpath discovery & use in AST Transforms

### Issue description

See https://github.com/apache/grails-core/pull/15669/files#r3344149649 - we should consider a grails.factory similar to the GSP, etc so that at compile time we would know all taglibs on the classpath

Start with the linked pull request discussion and the existing GSP grails.factory approach; the issue names no files or tests. Trace how compile-time AST transforms could discover taglibs on the classpath, then clarify the factory contract and expected behavior. Done means classpath taglibs are discoverable and usable by AST transforms, with tests for the agreed behavior.

Written by the indexing model from the issue text.