JavadocLinkGenerator does not yet support internalJavadocBaseUrls with module specific paths

### New feature, improvement proposal

For `externalJavadocBaseUrls` both the module specific (e.g. ) and the module agnostic path formats (e.g. ) are supported (being derived automatically from resources provided below the given baseUrls like `element-list` and `module-summary.html`).
However for `internalJavadocBaseUrls` this detection cannot be used as at the time of the descriptor generation the javadoc does not exist yet.

As a simple boolean flag is not enough since the plugin descriptor generation needs to figure out which package is contained in which module this requires some more logic (potentially leveraging the classpath and some reflection to check which module provides which package).

Start by tracing JavadocLinkGenerator and the plugin descriptor generation path mentioned in the issue. Compare how externalJavadocBaseUrls derives module-specific and module-agnostic paths, then determine how internal URLs can identify the module providing each package. Done means internalJavadocBaseUrls supports module-specific paths without requiring generated Javadoc resources to exist first.

Written by the indexing model from the issue text.