Add a blocklist / alternative feature

[theseer]

Over time, projects might become abandoned or turn out outright hostile - e.g. by using a typo name and/or claiming to be official when they, in fact, aren't.

Phive should have a means of prohibiting unwanted installations from happening - possibly with a warning that can be overruled to insall anyway - or by redirecting the user to the suggested alternative.

/cc @jrfnl