Add @glimmer/reference as a virtual package

[chancancode]

Another fix to #1487

The fix in #1495 turned out to be incomplete and didn't solve my actual problem (which I reduced away in my report). The actual import in the app was for @glimmer/reference, which internally imports from @glimmer/validator. I think because in @glimmer/reference's package.json it has its own dep on @glimmer/validator it escapes this rule and acquired the actual copy in node_modules, causing the duplication.

It seems like this would also be a problem if you, say, imported from @glimmer/runtime (which imports from @glimmer/reference and, in turns, @glimmer/validator)? I don't personally have that particular use case but I could see that happening. Do we just wait and see or is there something more targeted hard-coding that we should do specifically for @glimmer/validator?

[chancancode]

Failure seems unrelated

[ef4]

The more complete thing to do is to go through ember-source and capture the complete list of "packages" that it defines. Possibly per ember-source version. That would be the more reliable thing for us to use here.

For now this addition is good though. Thanks.