Add tests on FuchsiaUtils.getFilter()
Add Introspection interfaces for import-chain

Add Introspection interfaces for the components of the import-chain
(ImportationLinker, DiscoveryService and ImporterService).
Update tests using introspection functions
Allow ImporterService to throw ImporterException

Rename BadImportRegistration to ImporterService.
addImporterDeclaration and removeImportDeclaration can now throw
ImporterException.

Handle duplicate ImportDeclaration.
Handle duplicate ImportDeclaration into ADiscovery