Backport Avoid crashing for classpath elements that do not exists on the file system.

Added DummyVersion constant value, exists() to detect file that does not exists and getDummyVersion to provide a string version for a dummy file.
Implemented DummyVersion and isDummy in physical and node file systems.
Added simple ClasspathElementsTraverser tests using the PhysicalFileSystem.

Backport: Add `handleTopLvlJS` to ClasspathElementsTraverserTest.TestElementTraverser

(cherry picked from commit 76df98df86465cf7d1663226b3d6d1c20c33d006)

Conflicts:

	tools/shared/src/main/scala/scala/scalajs/tools/classpath/builder/ClasspathElementsTraverser.scala