<!--- Verify first that your issue is not already reported on GitHub -->
<!--- Also test if the latest release and devel branch are affected too -->
<!--- Complete *all* sections as described, this form is processed automatically -->

##### 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.

input_dir = collection_input[0]
    mock_display = MagicMock()
    monkeypatch.setattr(Display, 'vvv', mock_display)
    # This is expected to be ignored because it is in the root collection dir.
    release_file = os.path.join(input_dir, 'namespace-collection-0.0.0.tar.gz')
    # This is not expected to be ignored because it is not in the root collection dir.
    fake_release_file = os.path.join(input_dir, 'plugins', 'namespace-collection-0.0.0.tar.gz')
    for filename in [release_file, fake_release_file]:
        with open(filename, 'w+') as file_obj:
            file_obj.write('random')
            file_obj.flush()
    actual = collection._build_files_manifest(to_bytes(input_dir), 'namespace', 'collection')
    assert actual['format'] == 1
    plugin_release_found = False
    for manifest_entry in actual['files']:
        assert manifest_entry['name'] != 'namespace-collection-0.0.0.tar.gz'
        if manifest_entry['name'] == 'plugins/namespace-collection-0.0.0.tar.gz':
            plugin_release_found = True
    assert plugin_release_found
    assert mock_display.call_count == 1
    assert mock_display.mock_calls[0][1][0] == "Skipping '%s' for collection build" % to_text(release_file)
def test_build_ignore_symlink_target_outside_collection(collection_input, monkeypatch):