Warn if project sets crossScalaVersions

[bpholt]

I've been bitten by this a couple times over the years, when converting a project to use sbt-projectmatrix, forgetting to remove all the crossScalaVersions settings previously in the project definition. This feels like it's always an error, and the resulting issues are usually not obviously linked to this setting. Could we add something to the plugin to detect this situation and at least print a warning, or if it truly is always an error, fail to load the project until the setting is removed?

If this seems like something that would be merged, I'd plan to work on it and contribute a PR, but I wanted to check to see if it made sense before starting.

[eed3si9n]

Sounds good to me.