Friendly name package resolution fails if npm is not installed

[tevanoff]

Hello! An issue was recently reported on the Heroku CLI that we traced back to a subtle bug that was introduced in #283

That change assumes that the npm command exists on the system. If that's not the case, as is possible with some Heroku CLI installs that bundle a version of node, the friendly name resolution will silently fail. When this happens, it can result in the entirely wrong package being installed as a plugin, which can fail in various ways that are not apparent and are hard to debug.

Here's the CLI issue for more context: heroku/cli#2034

What do you think about explicitly raising an exception if npm does not exist to make the dependency clear? Or, would it make sense to fall back to the https version of the check to avoid the dependency when it's not necessary?

Thanks!

[mdonnalley]

@tevanoff Thanks for finding the issue. We don't have bandwidth to prioritize a fix for this now but we're always happy to review and merge PRs if you'd like to submit one