Merge AST extractor and source extractor

[pauxus]

Both extract javadoc from an AST node, so source extraction should just be a fallback step from AST extraction.

Refactor separate steps into services, which can be injected (SPI mechanism)

[pauxus]

This is actually not necessary. Since the source-extractor runs in an earlier compilation phase, when the AST extractor is used, the javadoc will already be placed in an annotation.

Still the naming should be improved.