Simplify plugin module reference in HasScalaPlugin trait

[Emin017]

• Remove import of build_.plugin.Plugin as it's no longer needed
• Directly reference plugin.cross(_scalaVersion) without type annotation

This fixes building Chisel when it’s used as nested build files:

❯ ./mill diplomacy.6_7_0.compile 2>&1
build.mill-61] compile compiling 1 Scala source to out/mill-build/compile.dest/classes ...
build.mill-61] [error] dependencies/chisel/build.mill:232:17
build.mill-61]   import build_.plugin.Plugin
build.mill-61]                 ^^^^^^
build.mill-61] value plugin is not a member of build_
build.mill-61]
build.mill-61] [error] one error found
66/71, 1 FAILED] ./mill diplomacy.6_7_0.compile 1s
build.mill-61] [error] compile Compilation failed

Contributor Checklist

• Did you add Scaladoc to every public function/method?
• Did you add at least one test demonstrating the PR?
• Did you delete any extraneous printlns/debugging code?
• Did you specify the type of improvement?
• Did you add appropriate documentation in docs/src?
• Did you request a desired merge strategy?
• Did you add text to be included in the Release Notes for this change?

Type of Improvement

• Bugfix

Desired Merge Strategy

• Squash: The PR will be squashed and merged (choose this if you have no preference).

Release Notes

Reviewer Checklist (only modified by reviewer)

• Did you add the appropriate labels? (Select the most appropriate one based on the "Type of Improvement")
• Did you mark the proper milestone (Bug fix: 3.6.x, 5.x, or 6.x depending on impact, API modification or big change: 7.0)?
• Did you review?
• Did you check whether all relevant Contributor checkboxes have been checked?
• Did you do one of the following when ready to merge:
  • Squash: You/ the contributor Enable auto-merge (squash) and clean up the commit message.
  • Merge: Ensure that contributor has cleaned up their commit history, then merge with Create a merge commit.