Make disallow-any-unimported flag invertible (#18030)

The `--follow-imports=skip` CLI option is pretty much unusable in a
project where `disallow_any_unimported` is set to true in the
configuration file, as this results in a large number of errors (due to
both flags being incompatible). We have a pretty standard project
configuration file (with `disallow_any_unimported = true` and
`follow_imports = 'normal'`), but for specific local development cases
where we want to run the mypy CLI with `--follow-imports=skip` it's
incredibly noisy due to the number of errors produced.

This change proposes making the `disallow-any-unimported` invertible, so
that the CLI can be used with `--follow-imports=skip` in a less noisy
way by using:
```bash
mypy --follow-imports=skip --allow-any-unimported path/to/my/file.py
```

Author: soceanainn

mypy/main.py

@@ -645,12 +645,6 @@ def add_invertible_flag(
         title="Disallow dynamic typing",
         description="Disallow the use of the dynamic 'Any' type under certain conditions.",
     )
-    disallow_any_group.add_argument(
-        "--disallow-any-unimported",
-        default=False,
-        action="store_true",
-        help="Disallow Any types resulting from unfollowed imports",
-    )
     disallow_any_group.add_argument(
         "--disallow-any-expr",
         default=False,
@@ -677,6 +671,12 @@ def add_invertible_flag(
         help="Disallow usage of generic types that do not specify explicit type parameters",
         group=disallow_any_group,
     )
+    add_invertible_flag(
+        "--disallow-any-unimported",
+        default=False,
+        help="Disallow Any types resulting from unfollowed imports",
+        group=disallow_any_group,
+    )
     add_invertible_flag(
         "--disallow-subclassing-any",
         default=False,

test-data/unit/check-flags.test

@@ -963,6 +963,12 @@ from missing import Unchecked
 
 t: Unchecked = 12  # E: Type of variable becomes "Any" due to an unfollowed import
 
+[case testAllowImplicitAnyVariableDefinition]
+# flags: --ignore-missing-imports --allow-any-unimported
+from missing import Unchecked
+
+t: Unchecked = 12
+
 [case testDisallowImplicitAnyGeneric]
 # flags: --ignore-missing-imports --disallow-any-unimported
 from missing import Unchecked