Implement :require [WIP]

[aherlihy]

#21658

[smarter]

This is pretty tricky. There is a method updateClassPath but it's not used anywhere and I'm pretty sure it never worked and was just copy-pasted from Scala 2.

ctx.platform.classPath is used from ctx.platform.rootLoader which is itself used to setup RootClass which corresponds to __root__ from which all imports start. But RootClass is a symbol created once when we initialize the initial context and never again, so updateClassPath won't actually help us import any new symbol

So I think we need to recreate a rootCtx, which is akin to what :settings seems to do:

scala3/compiler/src/dotty/tools/repl/ReplDriver.scala

Lines 542 to 543 in 5176f9f

	rootCtx = setupRootCtx(tokenize(arg).toArray, rootCtx)
	state.copy(context = rootCtx)

but I don't guarantee this works 😅