Make the generated subproject self-aware about its own virtual axes

[keynmol]

I thought that it would be useful for, say, defining project-wide settings that vary depending on the platform.

Having access to all of the axes can help separate the settings from the raw project matrix definition.

If this doesn't fit with projectmatrix vision, I'm happy not to pursue and just add the recommended approach (once I know it) to the documentation :)

TODO:

• Document the new task key

[eed3si9n]

So you want to make the subproject self-aware about their virtual axes? I think that's a great idea.

[eed3si9n]

My general preference is to ignore these using global gitignore - https://docs.github.com/en/free-pro-team@latest/github/using-git/ignoring-files#configuring-ignored-files-for-all-repositories-on-your-computer

[keynmol]

I will remove it from the PR, can always keep a local copy for myself :)

[eed3si9n]

Is this file necessary?

[keynmol]

Don't think so, I copypasted the one of the jvm ones I think :)

It's also present in two other projects:

./src/sbt-test/projectMatrix/jvm-sandwich-sbt-1.3/project/build.properties
./src/sbt-test/projectMatrix/jvm-sandwich/project/build.properties

I can remove those if you want?

[keynmol]

Apparently those are the only ones that were actually needed :D Kept them.

[eed3si9n]

Calling this projectAxes I think is confusing because we already have subproject axis. What do you think about calling it virtualAxes?

[keynmol]

Yep, will rename

[keynmol]

Seems like the test on native has been failing before these changes: https://github.com/sbt/sbt-projectmatrix/runs/1371093404

I'm gonna try to track it down with the lovely folks from SN: https://gitter.im/scala-native/scala-native?at=5fb676fbc10273610a15bdba

UPDATE: doesn't seem to be related to projectmatrix, I've reproduced it in scala-native/scala-native#2024 on a regular build