Fix files finding error if the glob result of excludes containing directory.

[linw1995]

Recently I used pdm build to build the source distribution file and found out it contains some un-required files such as cache files generated by mypy. So I used excludes = ["**/.mypy_cache"] from [tool.pdm] sections in pyproject.toml file. But it doesn't work at all.

I did some digging about the root cause of this problem. The _find_files_iter method uses exclude glob patterns to generate directories and files which need to be excluded from the distribution building. But the _match_path doesn't consider the scenario of pattern as a directory.

[frostming]

LGTM