<!--- 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.

elif not force:
                raise AnsibleError("- the directory %s already exists. "
                                   "You can use --force to re-initialize this directory,\n"
                                   "however it will reset any main.yml files that may have\n"
                                   "been modified there already." % to_native(obj_path))
        if obj_skeleton is not None:
            own_skeleton = False
            skeleton_ignore_expressions = C.GALAXY_ROLE_SKELETON_IGNORE
        else:
            own_skeleton = True
            obj_skeleton = self.galaxy.default_role_skeleton_path
            skeleton_ignore_expressions = ['^.*/.git_keep$']
        obj_skeleton = os.path.expanduser(obj_skeleton)
        skeleton_ignore_re = [re.compile(x) for x in skeleton_ignore_expressions]
        if not os.path.exists(obj_skeleton):
            raise AnsibleError("- the skeleton path '{0}' does not exist, cannot init {1}".format(
                to_native(obj_skeleton), galaxy_type)
            )
        template_env = Environment(loader=FileSystemLoader(obj_skeleton))
        
        if not os.path.exists(b_obj_path):
            os.makedirs(b_obj_path)
        for root, dirs, files in os.walk(obj_skeleton, topdown=True):
            rel_root = os.path.relpath(root, obj_skeleton)
            rel_dirs = rel_root.split(os.sep)
            rel_root_dir = rel_dirs[0]
            if galaxy_type == 'collection':
                
                in_templates_dir = rel_root_dir in ['playbooks', 'roles'] and 'templates' in rel_dirs
            else: