Allow to handle "no source root found" in a custom way

[TheElectronWill]

This introduce a "PathRecoverer" that can take action when a path is not in any source root. In particular, it can replace the path and allow to continue, or skip the element, or fail (and of course emit a warning, or whatever - but I didn't want to make the reporter itself handle logging, they will look better if handled from sbt, mill, etc.)

The next step is to use this in the sbt plugin, and then we can say goodbye to the "no source root found for $x" error!

[ckipp01]

Thanks a lot for the pr @TheElectronWill!

So my largest concern is compat. Right now these are breaking changes to public classes. So I think we might need to rethink this.

Secondly, on the reporting side, we really need to make sure that we complain loudly when we do come across this. If not, people will hit on this and just never report upstream issues when in reality, they are probably issues in the coverage instrumentation.

[ckipp01]

So I think we'll need to handle this a bit differently. These would be breaking changes for all the reporters, meaning another major bump, and I don't think we want to do another major atm. Can we potentially switch these up to either have a default implementation for this or another way that won't break compat for existing tools that are using this?

[TheElectronWill]

I understand.
I can make the default throw an exception (i.e. keep the old behavior) and allow the "recoverer" to be added up.

[ckipp01]

Nit, maybe personal preference so take it or leave it

Suggested change

	.find(root => canonicalSrc.startsWith(root))
	.map(root => canonicalSrc.substring(root.length))
	.collectFirst {
	case root if canonicalSrc.startsWith(root) =>
	canonicalSrc.substring(root.length)
	}

[TheElectronWill]

yeah I hesitated 😄

[TheElectronWill]

Secondly, on the reporting side, we really need to make sure that we complain loudly when we do come across this

How can the reporter emit a proper warning without having access to a logger? :c (or maybe it has access to something I've missed). I would like to avoid a basic println (which wouldn't be colored not formatted properly, and thus would be less likely to be seen IMO).

edit: wait, you mean the "reporting side" is actually "the sbt scoverage plugin" (and mill, gradle, etc.)? I planned to open a PR in it once this one is merged. By making the default throw an exception, and the plugins warn appropriately, it should be ok.