Remove #noqa from most common imports and add them to import_exceptions

We have a lot of import with #noqa that is there to ignore h302,
because it's traditional to import and use a name directly, instead
of a whole module. This hides other errors and gives people the
impression that it's actually fine to import non-modules, you just
have to slap #noqa on those lines.

I went through the code and identified about a dozen names that are
most commonly imported this way. I remove the #noqa tag from them,
and added them to the list in import_exceptions.

I also removed a few unused imports that were revealed in the process.

Change-Id: I27afb8e2b1d4759ec974ded9464d8f010312ee78