Engine restriction support with the multi-stage installer

[iarna]

There are two pieces to this:

First, we need to have a tree validator that looks for packages without valid engines and warns or errors (determined by --engine-strict and --force).

Second, the package version selector should enclude engine criteria in its package selector filter, UNLESS the package was specified on the command line and the specifier is *.

[iarna]

Closing this as fixed in npm/npm#multi-stage