Move pylint plugins to an installed package.

This means we can now run things like running pylint --version without having
to set PYTHONPATH correctly now.

When using skip_install=true, the plugins wouldn't work as they need to import
qutebrowser.

We have to keep setting PYTHONPATH in run_pylint_on_tests.py, otherwise we get
this error I don't quite understand:

    F:  1, 0: error while code parsing: Unable to load file
    '/home/florian/proj/qutebrowser/git/__init__.py' ([Errno 2] No such
    file or directory: '...') (parse-error)

.pylintrc

@@ -3,10 +3,10 @@
 [MASTER]
 ignore=resources.py
 extension-pkg-whitelist=PyQt5,sip
-load-plugins=pylint_checkers.config,
+load-plugins=qute_pylint.config,
-             pylint_checkers.modeline,
+             qute_pylint.modeline,
-             pylint_checkers.openencoding,
+             qute_pylint.openencoding,
-             pylint_checkers.settrace
+             qute_pylint.settrace
 
 [MESSAGES CONTROL]
 enable=all

scripts/dev/pylint_checkers/setup.py

@@ -0,0 +1,4 @@
+# This is only here so we can install those plugins in tox.ini easily.
+
+from setuptools import setup
+setup(name='qute_pylint', packages=['qute_pylint'])

scripts/dev/run_pylint_on_tests.py

@@ -60,7 +60,7 @@ def main():
     ]
 
     toxinidir = sys.argv[1]
-    pythonpath = os.environ['PYTHONPATH'].split(os.pathsep) + [
+    pythonpath = os.environ.get('PYTHONPATH', '').split(os.pathsep) + [
         toxinidir,
     ]
 

tox.ini

@@ -122,8 +122,6 @@ commands =
 [testenv:pylint]
 basepython = python3
 ignore_errors = true
-skip_install = true
-setenv = PYTHONPATH={toxinidir}/scripts/dev
 passenv =
 deps =
     {[testenv]deps}
@@ -131,6 +129,7 @@ deps =
     astroid==1.4.4
     pylint==1.5.4
     requests==2.9.1
+    ./scripts/dev/pylint_checkers
 commands =
     {envpython} scripts/link_pyqt.py --tox {envdir}
     {envpython} -m pylint scripts qutebrowser --output-format=colorized --reports=no
@@ -138,14 +137,13 @@ commands =
 
 [testenv:pylint-tip]
 basepython = python3
-skip_install = true
-setenv = {[testenv:pylint]setenv}
 passenv = {[testenv:pylint]passenv}
 deps =
     {[testenv]deps}
     {[testenv:misc]deps}
     hg+https://bitbucket.org/logilab/astroid
     hg+https://bitbucket.org/logilab/pylint
+    ./scripts/dev/pylint_checkers
 commands =
     {envpython} scripts/link_pyqt.py --tox {envdir}
     {envpython} -m pylint scripts qutebrowser --output-format=colorized --reports=no