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##### SUMMARY
The "source" folder of my collection contains several files that are not directly related to the collection itself. E.g. a `.gitignore`, a `.github` folder, a `Makefile` etc.
When using `ansible-galaxy collection build <folder>`, these files are included in the generated tarball. This is especially hurtful when the source folder also contains a Python venv that I use for developing.

##### ISSUE TYPE
- Bug Report

##### COMPONENT NAME
ansible-galaxy

##### ANSIBLE VERSION
<!--- Paste verbatim output from "ansible --version" between quotes -->
```paste below
ansible 2.9.0.dev0
  config file = /etc/ansible/ansible.cfg
  configured module search path = [u'/home/egolov/.ansible/plugins/modules', u'/usr/share/ansible/plugins/modules']
  ansible python module location = /home/egolov/Devel/ansible/ansible/lib/ansible
  executable location = /home/egolov/Devel/ansible/ansible/bin/ansible
  python version = 2.7.15 (default, Oct 15 2018, 15:26:09) [GCC 8.2.1 20180801 (Red Hat 8.2.1-2)]

```

##### CONFIGURATION
<!--- Paste verbatim output from "ansible-config dump --only-changed" between quotes -->
```paste below
empty
```

##### OS / ENVIRONMENT
Fedora


##### STEPS TO REPRODUCE
Build a collection :)

##### EXPECTED RESULTS
I'd expect only collection-relevant files/folders (those that are listed in the spec/docs) to be included.


##### ADDITIONAL INFORMATION
Alternatively, a ignore file as suggested by @bcoca  in https://github.com/ansible/ansible/pull/59121/files#r304554334 could be a fix for this.

headers = {}
            self.api._add_auth_token(headers, download_url, required=False)
            self.b_path = _download_file(download_url, b_temp_path, artifact_hash, self.api.validate_certs,
                                         headers=headers)
        if os.path.exists(b_collection_path):
            shutil.rmtree(b_collection_path)
        os.makedirs(b_collection_path)
        with tarfile.open(self.b_path, mode='r') as collection_tar:
            files_member_obj = collection_tar.getmember('FILES.json')
            with _tarfile_extract(collection_tar, files_member_obj) as files_obj:
                files = json.loads(to_text(files_obj.read(), errors='surrogate_or_strict'))
            _extract_tar_file(collection_tar, 'MANIFEST.json', b_collection_path, b_temp_path)
            _extract_tar_file(collection_tar, 'FILES.json', b_collection_path, b_temp_path)
            for file_info in files['files']:
                file_name = file_info['name']
                if file_name == '.':
                    continue
                if file_info['ftype'] == 'file':
                    _extract_tar_file(collection_tar, file_name, b_collection_path, b_temp_path,
                                      expected_hash=file_info['chksum_sha256'])
                else:
                    os.makedirs(os.path.join(b_collection_path, to_bytes(file_name, errors='surrogate_or_strict')))
    def set_latest_version(self):
        self.versions = set([self.latest_version])
        self._get_metadata()
    def _get_metadata(self):
        if self._metadata:
            return
        self._metadata = self.api.get_collection_version_metadata(self.namespace, self.name, self.latest_version)
    def _meets_requirements(self, version, requirements, parent):