<!--- 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, output_dir = collection_input
    existing_output_dir = os.path.join(output_dir, 'ansible_namespace-collection-0.1.0.tar.gz')
    os.makedirs(existing_output_dir)
    expected = "The output collection artifact '%s' already exists, but is a directory - aborting" \
               % to_native(existing_output_dir)
    with pytest.raises(AnsibleError, match=expected):
        collection.build_collection(input_dir, output_dir, False)
def test_build_existing_output_without_force(collection_input):
    input_dir, output_dir = collection_input
    existing_output = os.path.join(output_dir, 'ansible_namespace-collection-0.1.0.tar.gz')
    with open(existing_output, 'w+') as out_file:
        out_file.write("random garbage")
        out_file.flush()
    expected = "The file '%s' already exists. You can use --force to re-create the collection artifact." \
               % to_native(existing_output)
    with pytest.raises(AnsibleError, match=expected):
        collection.build_collection(input_dir, output_dir, False)
def test_build_existing_output_with_force(collection_input):
    input_dir, output_dir = collection_input
    existing_output = os.path.join(output_dir, 'ansible_namespace-collection-0.1.0.tar.gz')
    with open(existing_output, 'w+') as out_file:
        out_file.write("random garbage")
        out_file.flush()
    collection.build_collection(input_dir, output_dir, True)
    # Verify the file was replaced with an actual tar file
    assert tarfile.is_tarfile(existing_output)
@pytest.mark.parametrize('galaxy_yml', [b'namespace: value: broken'], indirect=True)
def test_invalid_yaml_galaxy_file(galaxy_yml):