FileResource and ClassPathEntry cleanup.

- Changes interning of FileResource to be thread-safe.
- Renames FileResource.create to FileResource.of as it
  doesn't always create.
- Drops the test-only needed getClassPathEntry from all
  Resource types.
- Removes unused findApplicableResources overload from all
  ClassPathEntry types.


Change-Id: I0d588dde8ae9220221c5f2255d01aa1feaba2c7b
Review-Link: https://gwt-review.googlesource.com/#/c/10830/