pylint: check for duplicate imports

[zenhack]

Pylint was complaining about this in 3 places:

• A duplicate import of paths
• Arguably a false positive, we were importing two different mock drivers locally, for the side-effects only, under the same name. I changed these to import the modules fully-qualified, which satisfies pylint.
• hil.model.db was being imported locally in the migration tests, in addition to being imported at the top of the file. I removed the redundant local import. While there, I also moved some local imports of non-extensions to global scope, as these are safe to import at the top of the file.

[naved001]

Looks good to me!

@djfinn14 can you review this?

@shwsun @knikolla ?

[zenhack]

Cool, merging.