Stops AST synthesizing the EntryMethodHolder.init() body.

Started creating with EntryMethodHolder.init() body via generator
instead of AST synthetisation. Resulted in deleting some redundant code
by reusing the GWT.create() handling already present in UnifyAst. The
resulting generated source files work for both monolithic and separate
compilation even though they handle GWT.create() calls differently,
whereas synthesizing AST would have needed two different
implementations.

Switches names of classes that are generated per-module, module names
stored in libraries, and name references to other libraries to be based
on the module's canonical name (to avoid collisions resulting from
multiple modules with the same rename).

Fleshes out the LibraryJavaToJavaScriptCompiler.ensureFullTypeLoaded()
call to match the correct type loading pattern of UnifyAst.findType(),
and comments on the desire to remove the duplication of logic.

Change-Id: I3d80aee451e6d1949d5746a77f4e096139386008
Review-Link: https://gwt-review.googlesource.com/#/c/6692/