Fix installation error from ARGV `--HEAD` filter

The ARGV filtering would strip `--HEAD` from dependency installations,
then proceed with trying to install the dependency from the latest
version in the formula. This causes an error if the latest version is
already installed, so check to see if formula is installed before
performing the installation.

Closes Homebrew/homebrew#8869.

Signed-off-by: Charlie Sharpsteen <source@sharpsteen.net>