Protect moduleNestedCtx to prevent it from being imported by import build._ (#5346)

Fixes #5345

The basic problem was that `import build._` pulled in the
`build.moduleNestedCtx` implicit, which depending on scoping could
over-ride the lexical `moduleNestedCtx`. This fix makes
`moduleNestedCtx` `protected`, so it is available in the nested scope
but cannot be imported

Tested manually, seems to fix the reported problem

Author: lihaoyi

core/define/src/mill/define/Module.scala

@@ -16,7 +16,7 @@ import scala.reflect.ClassTag
  * the concrete instance.
  */
 trait Module extends Module.BaseClass with ModuleCtx.Wrapper with ModuleApi {
-  implicit def moduleNestedCtx: ModuleCtx.Nested = moduleCtx
+  protected implicit def moduleNestedCtx: ModuleCtx.Nested = moduleCtx
     .withMillSourcePath(moduleDir)
     .withSegments(moduleSegments)
     .withEnclosingModule(this)