The logic to remove one Python module before installing another
only worked in simple cases where a file replaced a file.
- Installing a single file add-on over a Python package with the same
name caused an error as the directory isn't empty.
- Removing existing module directories from the zip-file did nothing
as the directories from the zip-file that end with a slash were
compared with directories from `os.listdir` that don't.
- `module_filesystem_remove` assumed ZipFile.namelist() was a list of
files in the root of the zip-file when it's a list of all files.
While I couldn't find any bugs caused by this, it performed checks
that don't make sense, comparing files at different depths of the
file-system.
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