Support custom matrices for artifact upload/download

[armanbilge]

Fixes #74. In doing so, I hope it fixes #81 as well.

This is a fairly complex change, consisting of the following:

1. Use all the keys in the matrix to determine the upload artifact id.

• Currently they are always $os-$scala-$java. If the matrix is crossed on other things, such as JVM/JS, this leads to artifacts with duplicate ids being uploaded. I suspect this may be what causes the failures we've been observing in CI.
• Now, they are named $os-$java-$scala-$key1-$key2 etc. This assumes that sensible value names are being used in the matrix (idk what the rules are for artifact upload names).

2. Introduce a githubWorkflowArtifactDownloadExtraKeys setting (please help bikeshed name).

• There are two kinds of keys wrt to the publishing step. Some keys (such as the Scala version) we must download artifacts for each value of that key. Other keys (such as OS or Java), we only need to download the artifacts for the primary value of that key.
• This setting is a Set[String] of keys of the first kind above (i.e., for which we must download artifacts for each value). We use it to add the key project (with values rootJVM, rootJS and/or rootNative) when setting up the platform cross in the matrix.
• With this information, the publish workflow "re-creates" the matrix as a sequence of steps for downloading artifacts, heeding any exclusions.

These changes are fairly internal but possibly deserve a minor version bump. In any case, before releasing I will test the snapshot on some relevant projects.

[rossabaker]

Regarding the choice of Set, does the order of keys matter, or do they just need to be in the same arbitrary order in order to generate a unique ID?

[armanbilge]

Just a consistent (but otherwise arbitrary) ordering. Maybe should be a SortedSet here then?

Edit: sorry, I'm confusing with the githubWorkflowArtifactDownloadExtraKeys setting which is a Set.

[rossabaker]

Okay, if consistent-but-arbitrary is the goal, and that makes sense, then I think it's fine.