Move linting related code to a new graylint package

[akaihola]

Running linters doesn't really fit together along with Darker's original purpose of executing code reformatters. Reformatters make local modifications to code, while linters only report rule violations and may detect flaws in one part of the code in relation to a different part (e.g. undefined identifiers).

Eventually we should remove -L / --lint from Darker completely, but before that we'd like to separate out linting functionality into a dedicated tool (named e.g. graylint) and move all related code there. Code needed by both Darker and that package should go into a third library support package (named e.g. libdarkgray). Darker can deprecate but still keep the -L / --lint option, and depend on graylint to implement it.

Alternatives considered:

• all code in the Darker repo, but in three different Python packages which are published separately on PyPI
• rename the Darker package to something more generic, and provide two console entry points darker and graylint in it
• duplicate all common code in both Darker and Graylint
• keep linting functionality in Darker, rename Darker to something more generic, add --black / --no-black and make --no-black the default, and re-create Darker as a thin wrapper with --black as the default

See #304 for discussion leading to this proposal.