Makefile: merge check-flake8 into check-package

Teach check-package to detect python files by type and check them using
flake8.
Do not use subprocess to call 'python3 -m flake8' in order to avoid too
many spawned shells, which in its turn would slow down the check for
multiple files. (make check-package takes twice the time using a shell
for each flake8 call, when compared of importing the main application)

Expand the runtime test and the unit tests for check-package.

Remove check-flake8 from the makefile and also from the GitLab CI
because the exact same checks become part of check-package.

Suggested-by: Arnout Vandecappelle <arnout@mind.be>
Signed-off-by: Ricardo Martincoski <ricardo.martincoski@gmail.com>
[Arnout: add a comment to x-python to explain its purpose]
Signed-off-by: Arnout Vandecappelle <arnout@mind.be>

utils/checkpackagelib/lib_python.py

@@ -0,0 +1 @@
+from checkpackagelib.tool import Flake8                # noqa: F401