Need to error out if a userenv req is not present in the "requirements" section

[atheurer]

Right now this completes without an error, and the user might assume the package is included when it is not.

[k-rister]

Hmm, this was supposed to be addressed here: https://github.com/perftool-incubator/workshop/blob/master/workshop.pl#L540

I guess I broke it somehow?