Viewcode: Fix issue with import paths that differ from the directory structure

[ProGamerGov]

This PR fixes an issue that occurs when, the module import path used by users does not match the directory / file structure. In this case, the import_lib's import_module function cannot find the correct path.

This issue results in code docs missing the associated viewcode link when the extension is used.

My simple solution utilizes importlib's __import__ function to find the actual directory module path to the import.

Feature / Bugfix

• Bugfix

Purpose

If the module import path used by users does not match the directory / file structure, import_lib's import_module function cannot find it.

Below is a basic example of where this failure occurs:

package/
    `__init__.py` # import package.subpackage1 as subpackage1
    subpackage1/
        `__init__.py` # from package.subpackage1.subpackage2 import module_1
        subpackge2/
            `__init__.py`
            module_1.py

Using the above directory / file structure, users import module1 via:

import package.subpackage1.module1

Sphinx is generally able to handle this sort of structure, except for the viewcode extension that uses the get_full_modname function from here: https://github.com/sphinx-doc/sphinx/blob/5.x/sphinx/util/__init__.py

That function specifically causes an error with the above example when importlib's import_module is run:

from importlib import import_module
module = import_module("package.subpackage1.module1") # Fails
# ModuleNotFoundError: No module named 'package.subpackage1.module1'

The exact path to the module however has no error:

from importlib import import_module
module = import_module("package.subpackage1.subpackage2.module1") # Works

Relates

A remake of this PR:
#10766

CC: @AA-Turner