Fix the scala-library dependency for (generic) platform modules

[lefou]

In Scala 3 modules, that also define a platform, the scala library is not correctly resolved.
The current implementation exists merely as a convenience for JS and Native modules. ScalaNativeModule already handles a correct scala library by overriding scalaLibraryIvyDeps. This pull request overrides the same in ScalaJSModule and removed the faulty platfrom dependency from ScalaModule.

E.g. imagine, someone tries to build a Mill plugin with Scala 3 (which is currently not supported) and sets the platform to _mill0.11, then you would run into the following issue:

object root extends ScalaModule {
  override def scalaVersion = "3.3.0"
  override def platformSuffix = "_mill0.11"
  // ..
}

>> mill ivyDepsTree
1 targets failed
resolvedIvyDeps
Resolution failed for 1 modules:
--------------------------------------------
  org.scala-lang:scala3-library_mill0.11_3:3.3.0
	not found: /Users/me/ivy2/local/org.scala-lang/scala3-library_mill0.11_3/3.3.0/ivys/ivy.xml
	not found: https://repo1.maven.org/maven2/org/scala-lang/scala3-library_mill0.11_3/3.3.0/scala3-library_mill0.11_3-3.3.0.pom

• See also Fix #1103: Scala 3 support for Scala.js #1187 (comment)

[lolgab]

Thank you for the fix! ❤️