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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.

dependencies, repository, documentation, homepage, issues, **kwargs):
    manifest = {
        'collection_info': {
            'namespace': namespace,
            'name': name,
            'version': version,
            'authors': authors,
            'readme': readme,
            'tags': tags,
            'description': description,
            'license': license_ids,
            'license_file': license_file if license_file else None,  
            'dependencies': dependencies,
            'repository': repository,
            'documentation': documentation,
            'homepage': homepage,
            'issues': issues,
        },
        'file_manifest_file': {
            'name': 'FILES.json',
            'ftype': 'file',
            'chksum_type': 'sha256',
            'chksum_sha256': None,  
            'format': MANIFEST_FORMAT
        },
        'format': MANIFEST_FORMAT,
    }
    return manifest
def _build_collection_tar(b_collection_path, b_tar_path, collection_manifest, file_manifest):