Fix #724: Slowness of source map resolution with CompleteIRClasspath.

Note that the source map resolution still *fails* to resolve anything.
This commit simply makes sure it fails *fast*.
Fix the nonsensical file name given to Rhino when loading lazily.

Now it refers to a file that does not actually exist, because it's
been generated in-memory, but at least it does not incorrectly
refer to an existing position in a Scala source file.
Deactivate source mapping when using a CompleteIRClasspath.

See #727, source mapping does not work with CompleteIRClasspath,
currently, so don't bother to try to resolve them.
Merge pull request #728 from sjrd/fix-source-mapping-slowness

Fix source mapping slowness