Support GitHub package versions with monorepos about tachometer

[Westbrook]

@aomarks I would love to see some sort of follow up to this.

Comparing to main or similar branch would be a great win, and if there is no other path to making that possible that some bash scripting, it would be super helpful to see how you saw that coming together!

Similarly, but not the same, I can easily implement testing against my current branch or against remote versions of the packages in my mono-repo, and it would seem like that Tachometer should be able to support comparing at least those two contexts without extra bash scripting. Being --package-version=next=@spectrum-web-components/button@current would already be using a temporary directory to run the test, could something in the realm --package-version=current allow for the test to run as if --package-version wasn't being used in order to get the comparison. Or maybe there's already a CLI option that I haven't been able to find that does that already?

It's possible that a single unifying approach to this sort of cross-version testing in a monorepo is a better goal than answering the two paths above separately, in which case I'd vote for a CLI path forward (if possible). Where bash scripting required here, I look forward to your thoughts on how you'd pursue that.

[aomarks]

This is published in v0.5.2, see https://github.com/Polymer/tachometer/blob/master/README.md#swap-npm-dependencies