Avoid trying to simultaneously install invalid peer dependencies.

Helps fix #3079: now an error will *never* be thrown, instead of the confusing ENOTEMPTY/ENOENT shitstorm that we saw in #3047.

It remains to make an error *always* be thrown, but this is a good first step. We should never try to install a package twice, concurrently, into the same place, even when there are conflicting peer dependencies.