Add JS dependency tracking via manifests.

A JS_DEPENDENCIES manifest file is added to each configuration
classpath and exported in JARs. This way, libraries may declare what
exact JS files should be included from a WebJar they depend on. (Or
what JS files they provide themselves should be included).

Example usages can be found in the build definitions for the
jasmine-test-framework and the testing example.

- This completely removes `importScripts` and fixes #457
- The project's classpath is fully available even in the post package
  stages. This is necessary to allow for discovery of dependency
  manifests and JS libraries the project depends on.
Merge pull request #579 from gzm0/js-deps

Add JS dependency tracking via manifests.