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 also completely removes `importScripts` and fixes #457.