Fix configuration of mypy plugins to point to paths not modules (apac…

…he#36563)

The configuration of our MyPy plugins was wrongly pointing to
modules rather than paths. This caused problems in the environment
where you had no PYTHONPATH set pointing to the root of your
Airflow sources. One of the side effects was that MyPy Plugin
for IntelliJ failed with "invalid plugin" error.

This PR changes the plugins to use relative paths instead - which
should work when mypy is invoked from the root of the
project (which in general is how our mypy gets invoked anyway and
is the default settings for most IDE integrations.

setup.cfg

@@ -206,8 +206,8 @@ no_implicit_optional = True
 warn_redundant_casts = True
 warn_unused_ignores = False
 plugins =
-  dev.mypy.plugin.decorators,
-  dev.mypy.plugin.outputs
+  dev/mypy/plugin/decorators.py,
+  dev/mypy/plugin/outputs.py
 pretty = True
 show_error_codes = True
 # Mypy since 0.991 warns about type annotations being present in an untyped