Check for collisions in Cross instances, ban forward slash in Cross segments, support os.SubPath in Cross modules

[jackkoenig]

Fixes #2835

This PR does 3 things (and I'm happy to split it into smaller PRs since each commit is separable):

1. Checks for collisions in cross instances
2. Bans '/' in cross segments (my original approach is now disallowed, and the user is encouraged to use os.SubPath)
3. Adds support for os.SubPath in Cross instances

My implementations for (1) and (2) could be better, I don't love throwing exceptions, but I wanted to get something functional out for review to get pointers on how better to report errors to the user here. It would be nice to print the actual line rather than just printing the filename and line.

For (3), I have not yet added a test because I figure it would make sense to include in the examples, probably in a section immediately after https://mill-build.com/mill/Cross_Builds.html#_dynamic_cross_modules (ie. before Use Case: Static Blog), but I wanted @lefou's blessing before starting that work.

[lefou]

In #2835 (comment) I wrote:

I don't think allowing a cross-value to open up multiple sub-paths is the right solution. We already map n cross-values to n sub-directories. Now, when we also allow the value itself to contain sub-directories, we can't no longer guarantee unique T.dest directories.

Here is my counter example, in which both cross module instances whould use the same T.dest path for their targets.

import mill._
object foo extends Cross[Foo](("a", "b/c"), ("a/b", "c"))
trait Foo extends Cross.Module2[String, String] {
  ...
}

Instead, we could encode the special / character in the path. But that would create less nice cache directory names.

[lefou]

Please change it to os.SubPath to avoid escaping the base path with ... Also, as discussed in #2835, we need to check the given cross instances for collisions (at runtime).

[jackkoenig]

@lefou I ended up taking a slightly different approach, rather than using os.SubPath when processing segments, I banned / in segments and instead encourage the user to use os.SubPath themselves. See the 1st PR comment up top where I've described the new approach in detail.

[lefou]

I'm not convinced. Why we want to force a path on things that aren't paths, like a version matrix? As we agreed that we want cross module parameters to be able to take more that one segment, why should we only allow that for paths, but not other values? Instead we should encourage to model a build using domain types when needed. This includes avoiding generic data types like strings and paths in favor to more specific types, hence the type class to still be able to map to a segmented path.

IMHO, the essential thing is to make sure, there are no collisions, so we have safe cache locations and unambiguous CLI arguments.

[lefou]

Superseded by #2984