Detect if Composer is installed or not

If Composer isn't installed, you'll see this:

sh: 1: composer: not found [notice]

...so we added it to the requirements list. But we should check if it's there, as this error doesn't even come up unless --debug is enabled.

ergonlogic: in Aegir5, I install drush 8.x into /var/aegir/.local/bin so we could do something similar here

helmo42: We don't have a hook for requirements checking in the backend ... that might be a nice addition to the concept of a module. Maybe a server verify hook could check it and set a flag that then gets shown via Drupal's normal hook_requirements

ergonlogic: We could run "which composer" in the backend, and flag a warning/error if it comes up empty.