[emersonknapp]

Fix two polymath-ansible bugs and release 2.7.1.

• Put ansible/collections/ back on the collections path ahead of the .polymath-ansible/ cache.
  Since 2.3.0, playbooks that use a collection the repo develops itself failed syntax-check with "not found", unless the playbook sat beside a collections/ directory.
• Run the hook as a single process with require_serial.
  Under --all-files, several processes installed ansible/requirements.yml into the same cache at once, and ansible-galaxy failed with "Directory not empty".
• Bump to 2.7.1 with a changelog entry.

A new test lints a nested playbook that uses a role from the repo's own collection, and fails without the fix.