Robustly discover packages with @manypkg

[NullVoxPopuli]

Resolves: #46
Requires: embroider-build/github-changelog#8 (included in this PR, as it is one half of the solution to #46)

Can test on your own repo with an override:

"pnpm": {
  "overrides": {
    "release-plan": "github:embroider-build/release-plan#tmp-manypkg-dist"
  }
}

Additionally, this should unlock the ability to remove --singlePackage, as @manypkg just works.

[mansona]

As discussed in the tooling meeting: we should add some basic tests here before proceeding

[NullVoxPopuli]

As discussed in the tooling meeting: we should add some basic tests here before proceeding

done -- I added two tests for some scenarios that I'm most immediately concerned with.

[mansona]

Looks good! thanks

[mansona]

I marked this as an enhancement because it changes the internal implementation of something. Really it's a bugfix because it's trying to fix a specific bug but it's too much of a big change to just be considered a bugfix 👍

edit: i forgot that we are pre-1.0 so actually an "enchancement" is really a bugfix 😭 maybe this is something that we should codify in release-plan