show the library a symbol resolves to

[devoncarew]

When we show symbol usage, we just show the name of a symbol (createDriver). This symbol may not be unique in the context of a package however. We should show the library that the symbol was defined in (or perhaps just show it to disambiguate symbols?).

ex., package:webdriver has createDriver symbols in lib/async_core.dart, lib/async_html.dart, lib/async_io.dart, lib/sync_core.dart, and lib/sync_io.dart.