In the current pylint handling, the only way to use an alternative
config is to explicitly pass it with the "--rcfile" flag, which is
confusingly inconsistent with normal pylint config file resolution.
This change allows local pylintrc files or PYLINTRC env var to override
the default depot_tools config, which is often desirable to enforce
different style guidelines for different projects.

For reference, the config file search order[1] is:
1) File specified by the "--rcfile" flag
2) pylintrc in the CWD
3) .pylintrc in the CWD
4) first pylintrc found up the python module hierarchy, if CWD is in
   a module hierarchy
5) $PYLINTRC env var
6) $HOME/.pylintrc
7) $HOME/.config/pylintrc
8) /etc/pylintrc (or equivalent on other platforms)

- 1-5 will take precedence over the new depot_tools default.
- If there is no match for 1-5 and there is no PYLINTRC env var,
  depot_tools will set PYLINTRC to its default and that will be used.
- If there is no match for 1-5 and the PYLINTRC env var is empty, then
  it will fall through to 6-8.

[1] Newer pylints have additional options, but these are the options,
and the order, common to all versions of depot_tools pylint.

Change-Id: Ib725b15bb639dc9c7cb9009fd3b504124e0c1f2a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2708749Reviewed-by: Mike Frysinger <vapier@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@google.com>
Commit-Queue: Michael Moss <mmoss@chromium.org>