tests: Rename test_overrides to test_import_machinery

Rename this test to better suit the contents of the file. This also gives
us a more concise location to grow other tests related to import machinery
internals.

https://bugzilla.gnome.org/show_bug.cgi?id=726877
Raise ImportError when importing modules not found in repository

Raise an ImportError with extra information noting the typelib was not
found. This removes the need to log a similar message which adds output
noise when attempting controlled imports within try/except statements.
In Python 2, the additional information is lost but in Python 3 it shows up.

https://bugzilla.gnome.org/show_bug.cgi?id=726877