Inconsistent module import behavior when `follow_imports=skip` and __init__ files are omitted from check list

Bug Report

Inconsistent module import behavior when follow_imports=skip and files are passed explicitly on the command line

To Reproduce

mypy.ini

[mypy]
follow_imports = skip

a.py

import mod.b
reveal_type(mod.b.UserId)
from mod.b import UserId
reveal_type(UserId)
import sib
reveal_type(sib.Sib)
from sib import Sib
reveal_type(Sib)

sib.py

class Sib:
    pass

mod/b.py

class UserId:
    pass

and a blank mod/__init__.py

nipunn-mbp:mypy_repro nipunn$ mypy a.py sib.py mod/b.py
a.py:2: note: Revealed type is 'Any'
a.py:4: note: Revealed type is 'def () -> mod.b.UserId'
a.py:7: note: Revealed type is 'def () -> sib.Sib'
a.py:9: note: Revealed type is 'def () -> sib.Sib'

nipunn-mbp:mypy_repro nipunn$ tree .
.
├── a.py
├── mod
│   ├── __init__.py
│   └── b.py
├── mypy.ini
└── sib.py
1 directory, 5 files

notably, you get the expected behavior when __init__.py is passed explicitly

nipunn-mbp:mypy_repro nipunn$ mypy .
a.py:2: note: Revealed type is 'def () -> mod.b.UserId'
a.py:4: note: Revealed type is 'def () -> mod.b.UserId'
a.py:7: note: Revealed type is 'def () -> sib.Sib'
a.py:9: note: Revealed type is 'def () -> sib.Sib'

nipunn-mbp:mypy_repro nipunn$ mypy a.py sib.py mod/b.py mod/__init__.py
a.py:2: note: Revealed type is 'def () -> mod.b.UserId'
a.py:4: note: Revealed type is 'def () -> mod.b.UserId'
a.py:7: note: Revealed type is 'def () -> sib.Sib'
a.py:9: note: Revealed type is 'def () -> sib.Sib'

Expected Behavior

It appears inconsistent that mypy would believe in mod.b.UserId while not believing in mod.b under these circumstances - giving different behavior for UserId depending on how it is imported. Even with follow_imports=skip - I would expect __init__.py files to be followed when a file within is explicitly declared.

Start by running the reproduction with mypy.ini, a.py, sib.py, mod/b.py, and mod/init.py, then compare the explicit file lists shown in the report. Trace how explicitly checked modules and package init.py files are handled with follow_imports=skip. Done means the revealed types are consistent whether mod/init.py is listed explicitly or not.