Unable to type check only specific submodule/subpackage.

Bug Report

Perhaps, this is a "skill issue" on my side, but I was unable to figure out how to make mypy type check a specific submodule/subpackage only. Documentation and previous issues on this topic recommend using the --follow-imports flag, but this flag doesn't quite solve the problem, and it seems to be more of a workaround than a true solution.

To Reproduce

Consider the following source tree:

# foo/__init__.py
from . import _internal_junk, strictly_typed_public_module

# foo/_internal_junk/__init__.py
from . import junk

# foo/_internal_junk/junk.py
def incorrectly_typed_function(val: int) -> str: return val / 2.0

# foo/strictly_typed_public_module/__init__.py
from . import api_component1, api_component2

# foo/strictly_typed_public_module/api_component1.py
from some_different_package import this_package_has_a_typing_error

def this_is_ok() -> str: return "okay"

# foo/strictly_typed_public_module/api_component2.py
def this_mistake_should_be_caught_by_mypy(v: str) -> int: return 2.0

# some_different_package.py
def this_package_has_a_typing_error(a: str) -> int: return a

Then, here are some of my attempts to check only the strictly_typed_module submodule:

mypy --strict -m foo.strictly_typed_public_module
mypy --strict -p foo.strictly_typed_public_module
mypy --strict    foo/strictly_typed_public_module

mypy --strict -m foo.strictly_typed_public_module  --follow-imports=silent

mypy --strict    foo/strictly_typed_public_module  --follow-imports=silent
mypy --strict -p foo.strictly_typed_public_module  --follow-imports=silent

None of these produce the desired results.

Expected Behavior

1. mypy should find all the typing problems in the strictly_typed_public_module submodule:
   1. type check all the files that are part of this submodule
   2. transitively follow all imports in those files
2. mypy should NOT check any unrelated files that are not submodules of the strictly_typed_public_module OR imported by it.

Actual Behavior

The first 3 commands (without --follow-imports) all produce the same output:

foo/_internal_junk/junk.py:1: error: Incompatible return value type (got "float", expected "str")  [return-value]
some_different_package.py:1: error: Incompatible return value type (got "str", expected "int")  [return-value]
foo/strictly_typed_public_module/api_component2.py:1: error: Incompatible return value type (got "float", expected "int")  [return-value]
Found 3 errors in 3 files (checked 1 source file)

Here, mypy checks foo/_internal_junk/junk.py even though it was not requested on the CLI, and it's not imported by the strictly_typed_public_module.

The next command (-m with --follow-imports=silent) just ignores all imports and seemingly only checks the single __init__.py file:

Success: no issues found in 1 source file

The final 2 commands ALMOST do the right thing:

foo/strictly_typed_public_module/api_component2.py:1: error: Incompatible return value type (got "float", expected "int")  [return-value]
Found 1 error in 1 file (checked 3 source files)

Except that they also ignore the external import from some_different_package.

Your Environment

• Mypy version used: 1.5.1 (compiled: yes)
• Mypy configuration options from mypy.ini (and other config files): None
• Python version used: 3.11.3

Start by reproducing the documented -m, -p, path, and --follow-imports commands against the example package tree in the issue. Trace how mypy selects package files and follows imports; done means the selected submodule and its transitive imports are checked while unrelated modules are excluded.