We need to install the helper module not just on `etc` but also on all
subdirectories of it too. Also, handle the case where we install
a subdirectory with etc.install.

Closes Homebrew/homebrew#26145.

I've given up on this; too many edge cases.
If someone else wants to pick it up then send me a message and I'll walk
you through the approach.

This method allows copying a file to a new location by performing a
substitution on the pathname.

If an etc file exists on installation instead of overwriting it (or
requiring all the manual checks in formula) simply copy it with the
extension `.default` appended.