I think it may be the same question with #12880

Edit:
It's more difficult for this one,

• os.path.normcase always return lowercase
• os.path.normpath do not normalize case
• pathlib.Path(...).resolve() will return the abs path, and make drive name uppercase, but do not change the case of directory

It seems no simple solution for this one.

This may help #12880:
replace

path = os.path.normpath(path)
drive, root, path = os.path.splitroot(path)
if drive.endswith(":"):
    drive = drive.upper()
path = os.path.join(drive, root, path)

I was able to replicate this issue.

Here are the examples translated into cmdline.test tests:

[case testCmdlinePackageWindowsCaseBaseCase]
# cmd: mypy .\ModuleName\main.py
[file ModuleName/__init__.py]
[file ModuleName/main.py]
from .file import *
[file ModuleName/file.py]

[case testCmdlinePackageWindowsCase]
# cmd: mypy .\moduleName\main.py
[file ModuleName/__init__.py]
[file ModuleName/main.py]
from .file import *
[file ModuleName/file.py]

(Both of them "should" pass, but only the first one does.)

I guess you would have to make sure they only run on Windows; I don't know how to do that.