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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.

except YAMLError as err:
        raise AnsibleError("Failed to parse the galaxy.yml at '%s' with the following error:\n%s"
                           % (to_native(b_galaxy_yml_path), to_native(err)))
    set_keys = set(galaxy_yml.keys())
    missing_keys = mandatory_keys.difference(set_keys)
    if missing_keys:
        raise AnsibleError("The collection galaxy.yml at '%s' is missing the following mandatory keys: %s"
                           % (to_native(b_galaxy_yml_path), ", ".join(sorted(missing_keys))))
    extra_keys = set_keys.difference(all_keys)
    if len(extra_keys) > 0:
        display.warning("Found unknown keys in collection galaxy.yml at '%s': %s"
                        % (to_text(b_galaxy_yml_path), ", ".join(extra_keys)))
    
    for optional_string in string_keys:
        if optional_string not in galaxy_yml:
            galaxy_yml[optional_string] = None
    for optional_list in list_keys:
        list_val = galaxy_yml.get(optional_list, None)
        if list_val is None:
            galaxy_yml[optional_list] = []
        elif not isinstance(list_val, list):
            galaxy_yml[optional_list] = [list_val]
    for optional_dict in dict_keys:
        if optional_dict not in galaxy_yml:
            galaxy_yml[optional_dict] = {}
    
    galaxy_yml['license_ids'] = galaxy_yml['license']
    del galaxy_yml['license']
    return galaxy_yml
def _build_files_manifest(b_collection_path, namespace, name):