Implement requirePrefix() on ChildSourceAssetLocation, as this will need
to give a different result, as the 'src' directory is not part of the
require path.
Adding an specific exception for completely invalid require paths, which
will be needed once we start enforcing class namespaces.
Start returning different asset locations for BladeRunnerJS third-party
libraries.
Fixed bug in ChildSourceAssetLocation.requirePrefix().
Converted test I missed earlier to use 'appns' rather than 'mypkg'.