If plugin has same name as top-level module which has been imported before mypy runs (e.g., you are using api), mypy picks up the wrong file

Bug Report

In some circumstances, a plugin which works fine from command line fails when run from the API with:

('Found 1 error in 1 file (errors prevented further checking)\n', '/data/users/ezyang/b/pytorch/mypy.ini:5:1: error: Plugin "/data/users/ezyang/b/pytorch/test/typing/../../mypy_plugins/sympy.py" does not define entry point function "plugin"  [misc]\n', 2)

The problem appears to occur when the a filepath plugin is provided which has the same name as a top level module that has already been imported when mypy runs.

To Reproduce

1. Create a plugin mypy_plugins/sympy.py, any plugin code is fine, it doesn't matter (you will never hit/import the plugin code)
2. Direct mypy to use the plugin plugins = mypy_plugins/sympy.py
3. Run mypy via the API on some test file (doesn't matter):

from mypy import api
import sympy

print(api.run(['--config-file', 'mypy.ini', 'test.py']))

Expected Behavior Plugin loads successfully, as it does when you call mypy directly

Actual Behavior Fails with "/data/users/ezyang/b/pytorch/test/typing/../../mypy_plugins/sympy.py" does not define entry point function "plugin"

Your Environment

• Mypy version used: 1.8.0
• Python version used: Python 3.10

Reproduce the failure by importing sympy before calling mypy.api.run with mypy.ini and mypy_plugins/sympy.py, then compare it with a direct mypy invocation. Trace how the filepath plugin is resolved when a same-named top-level module is already imported; done means both invocation paths load the plugin successfully and the reported entry-point error is gone.