<!--- 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.

namespace, name = collection.split('.', 1)
        galaxy_meta = None
        for api in apis:
            try:
                if not (requirement == '*' or requirement.startswith('<') or requirement.startswith('>') or
                        requirement.startswith('!=')):
                    if requirement.startswith('='):
                        requirement = requirement.lstrip('=')
                    resp = api.get_collection_version_metadata(namespace, name, requirement)
                    galaxy_meta = resp
                    versions = [resp.version]
                else:
                    resp = api.get_collection_versions(namespace, name)
                    
                    
                    versions = [v for v in resp if StrictVersion.version_re.match(v)]
            except GalaxyError as err:
                if err.http_code == 404:
                    display.vvv("Collection '%s' is not available from server %s %s"
                                % (collection, api.name, api.api_server))
                    continue
                raise
            display.vvv("Collection '%s' obtained from server %s %s" % (collection, api.name, api.api_server))
            break
        else:
            raise AnsibleError("Failed to find collection %s:%s" % (collection, requirement))
        req = CollectionRequirement(namespace, name, None, api, versions, requirement, force, parent=parent,
                                    metadata=galaxy_meta)
        return req
def build_collection(collection_path, output_path, force):