Most of the problems with our logic were due to not having the same
logic as our exclude parameter. This refactors that out into a separate
function so we can confidently achieve that.

Instead of just using Flake8 and pylint to keep Flake8 clean, let's also
use black to make it less manual for clean-up.

Resolve "flake8 does not generate error when given a non-existent file on the command line"

Closes #405

See merge request pycqa/flake8!227

This includes creating the pool, tearing it down under normal use, and
tearing it down in case of exception.

Doing this makes it harder to leak processes, as for instance was
happening in #410.

Fixes #410

This handles most any exception arising from a plugin and provides the
user with a better experience and view of the problem. It also provides
a way to retrieve the information about what exactly failed for
providing bug reports to plugin authors.

When using the `--diff` option flake8 only lints *.py files or files
defined by the `--filename` filter.

Fixes #362

* Use set literals instead of set([...])
* Avoid list(sorted(...)) as sorted returns a list
* Replace dict() with dict literal

When we receive a SyntaxError, it is not guaranteed to have a token that
includes the physical line causing the issue. If it does not, we now
will avoid trying to determine the number of rows and columns that are
actually there and default to what the error tells us.

Closes #279