Cannot specify srcjars as srcs for javadoc_library

In google/elemental2, most Java sources are actually generated by google/jsinterop-generator (`jsinterop_generator` rules), and I can't seem to generate javadoc for those using e.g. `srcs = [":libcore-src.jar"]`

Actually, it looks like `srcs` is only used to compute whether the rule is uptodate or needs to be re-run, and `root_packages` and `exclude_packages` are the _real_ inputs to the `javadoc` command.