[lit] Remove (or allow specific) unused imports

Summary:
Using Python linter flake8 on the utils/lit reveals several linter
warnings designated "F401: Unused import". Fix or silence these
warnings.

Some of these unused imports are legitimate, while some are part of lit's API.
For example, users of lit expect to be able to access `lit.formats.ShTest` in
their `lit.cfg`, despite the module hierarchy for that symbol actually being
`lit.formats.shtest.ShTest`. To silence linter errors for these lines,
include a "noqa" directive.

Reviewers: echristo, delcypher, beanz, ddunbar

Subscribers: mehdi_amini, llvm-commits

Differential Revision: https://reviews.llvm.org/D25407

llvm-svn: 283710