Discriminate loaded extensions using their alias

Discriminate loaded extensions using their alias

This patch simply changes the logic for discriminating which
extensions have already been loaded using the extension alias
rather than the module name.

This will avoid failures already observed in unit tests occurring
if two extension modules happen to have the same module file name
even if they're unrelated.

This change cannot be unit-tested as the neutron unit test
framework is currently lacking a mechanism to mock and validate
extension loading from the file system.

Closes-Bug: #1417180

Change-Id: Ia214a7c5b1127dbbec3c81023fdbc48d7fadf18f