Update javax.lang.model APIs

 we should make sure our modeling APIs work reasonably when used from a Plugin or through JavacTask. Esp. Elements.getPackageElement/getTypeElement in multi-module mode and with -source < 9 (getPackageElement, and presumably getTypeElement, does not currently work with -source < 9, as getPackageElement uses unnamed module, while -source < 9 runs in "noModule" mode, causing a NPE in Symtab ).