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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.

self.namespace = namespace
        self.name = name
        self.b_path = b_path
        self.api = api
        self.versions = set(versions)
        self.force = force
        self.skip = skip
        self.required_by = []
        self._metadata = metadata
        self._files = files
        self.add_requirement(parent, requirement)
    def __str__(self):
        return to_native("%s.%s" % (self.namespace, self.name))
    def __unicode__(self):
        return u"%s.%s" % (self.namespace, self.name)
    @property
    def latest_version(self):
        try:
            return max([v for v in self.versions if v != '*'], key=LooseVersion)
        except ValueError:  
            return '*'
    @property
    def dependencies(self):
        if self._metadata:
            return self._metadata.dependencies
        elif len(self.versions) > 1:
            return None
        self._get_metadata()
        return self._metadata.dependencies
    def add_requirement(self, parent, requirement):