Make --explicit-package-bases invertible (#9969)

Closes #9968

This PR adds the inverse of --explicit-package-bases: --no-explicit-package-bases.
The flag can be used if you want to override explicit_package_bases = True which was 
set in a configuration file.

Author: gustavgransbo

mypy/main.py

@@ -805,9 +805,10 @@ def add_invertible_flag(flag: str,
         title="Running code",
         description="Specify the code you want to type check. For more details, see "
                     "mypy.readthedocs.io/en/stable/running_mypy.html#running-mypy")
-    code_group.add_argument(
-        '--explicit-package-bases', action='store_true',
-        help="Use current directory and MYPYPATH to determine module names of files passed")
+    add_invertible_flag(
+        '--explicit-package-bases', default=False,
+        help="Use current directory and MYPYPATH to determine module names of files passed",
+        group=code_group)
     code_group.add_argument(
         "--exclude",
         metavar="PATTERN",