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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.

files_manifest_json = to_bytes(json.dumps(file_manifest, indent=True), errors='surrogate_or_strict')
    collection_manifest['file_manifest_file']['chksum_sha256'] = secure_hash_s(files_manifest_json, hash_func=sha256)
    collection_manifest_json = to_bytes(json.dumps(collection_manifest, indent=True), errors='surrogate_or_strict')
    with _tempdir() as b_temp_path:
        b_tar_filepath = os.path.join(b_temp_path, os.path.basename(b_tar_path))
        with tarfile.open(b_tar_filepath, mode='w:gz') as tar_file:
            
            for name, b in [('MANIFEST.json', collection_manifest_json), ('FILES.json', files_manifest_json)]:
                b_io = BytesIO(b)
                tar_info = tarfile.TarInfo(name)
                tar_info.size = len(b)
                tar_info.mtime = time.time()
                tar_info.mode = 0o0644
                tar_file.addfile(tarinfo=tar_info, fileobj=b_io)
            for file_info in file_manifest['files']:
                if file_info['name'] == '.':
                    continue
                
                filename = to_native(file_info['name'], errors='surrogate_or_strict')
                b_src_path = os.path.join(b_collection_path, to_bytes(filename, errors='surrogate_or_strict'))
                def reset_stat(tarinfo):