the search for `version.py` should exclude `exclude-paths` specified in `release-please-config.json` especially for the `.` path

The search for `version.py` [here](https://github.com/googleapis/release-please/blob/3569f158158e2bd096a94c145bdb1a7f455829f0/src/strategies/python.ts#L122-L124) doesn't consider `exclude-paths`. It simply searches all subfolders, and that is problematic for the package with the `.` path because all sub-packages will get a version update when the root package is updated.

Below is an example set-up where this is an issue, i.e. `/packages/xyz/src/xyz/version.py` shouldn't be updated because it's part of the `excluded-paths`.

### folder structure
```
/src/abc/version.py
/packages/xyz/src/xyz/version.py
```
### release-please-config.json
```json
"packages": {
".": {
"package-name": "abc",
"release-type": "python",
"exclude-paths": ["packages"]
},
"packages/xyz": {
"package-name": "xyz",
"release-type": "python"
}
},
```