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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.

n_filename = to_native(filename, errors='surrogate_or_strict')
    try:
        member = tar.getmember(n_filename)
    except KeyError:
        raise AnsibleError("Collection tar at '%s' does not contain the expected file '%s'." % (to_native(tar.name),
                                                                                                n_filename))
    with tempfile.NamedTemporaryFile(dir=b_temp_path, delete=False) as tmpfile_obj:
        bufsize = 65536
        sha256_digest = sha256()
        with _tarfile_extract(tar, member) as tar_obj:
            data = tar_obj.read(bufsize)
            while data:
                tmpfile_obj.write(data)
                tmpfile_obj.flush()
                sha256_digest.update(data)
                data = tar_obj.read(bufsize)
        actual_hash = sha256_digest.hexdigest()
        if expected_hash and actual_hash != expected_hash:
            raise AnsibleError("Checksum mismatch for '%s' inside collection at '%s'"
                               % (n_filename, to_native(tar.name)))
        b_dest_filepath = os.path.join(b_dest, to_bytes(filename, errors='surrogate_or_strict'))
        b_parent_dir = os.path.split(b_dest_filepath)[0]
        if not os.path.exists(b_parent_dir):
            
            
            os.makedirs(b_parent_dir)
        shutil.move(to_bytes(tmpfile_obj.name, errors='surrogate_or_strict'), b_dest_filepath)