Cleanup mill.define.Module and mill.define.Ctx

[lihaoyi]

• Bundle the necessary things we want to propagate together under Ctx.Nested, rather than passing a loose bunch of implicits. This reduces the amount of implicits we have flying around, and lets us easily mark fields on Ctx.Nested as private where possible so we can add or remove them

• Standardize naming convention of module members as module*: moduleDirectChildren, moduleCtx, etc. Fixes Rename millSourcePath #4111 by renaming millSourcePath to moduleDir

• Clean up mill.define.Ctx, removing a bunch of the special case semantics around RootModule/SubfolderModule

• Add some basic unit tests in core/define/test/src/mill/define/ModuleTests.scala to assert on the module instantiation and context propagation, without the complexity of downstream tests in exec or resolve

• Introduced mill.define.internal package, which is stuff in mill.define that isn't really intended for direct use. Some of it is private but needs to be in this module, some of it can't be private but isn't really meant to be used directly.