First pass at splitting out worker-api from mill core. (#504)

This reduces the {scala,scalajs,scalanative}-worker dependency from the entirety of Mill to a much narrower `mill.api` module. This reduces the amount of classpath pollution within these workers, should mean they're much faster to download the first time, and reduces the amount of random junk they would pull in if they were to be used outside of the Mill project.

The interactions between the various *Modules and their *WorkerImpls has been narrowed down to the `*.api` modules, which only depend on other `*.api` modules.

A lot of things have been moved around; user code is unlikely to break, but it's possible some will if it references classes that have been moved around. Forwarders have been left for the few internal classes that Mill uses in it's own `build.sc`, to support bootstrapping. Third-party code which breaks should be a straightforward to fix just by updating imports

The `*.api` modules have minimal dependencies (mostly uPickle and os-lib) and minimal code. There is still a bunch of implementation code in there: some of it defining data-types that are commonly sent across the module/worker interface (`Agg`, `PathRef`, ...), and some of it just general helper functions that are needed both in modules and workers. The latter code isn't strictly API definitions, but for now is small enough it's not worth splitting into it's own module

4 files changed, 5 insertions, 5 deletions

diff --git a/integration/test/resources/ammonite/build.sc b/integration/test/resources/ammonite/build.sc
index 66e67087..86f354b4 100644
--- a/integration/test/resources/ammonite/build.sc
+++ b/integration/test/resources/ammonite/build.sc
@@ -280,7 +280,7 @@ def generateConstantsFile(version: String = buildVersion,
                           unstableCurlUrl: String = "<fill-me-in-in-Constants.scala>",
                           oldCurlUrls: Seq[(String, String)] = Nil,
                           oldUnstableCurlUrls: Seq[(String, String)] = Nil)
-                         (implicit ctx: mill.util.Ctx.Dest)= {
+                         (implicit ctx: mill.api.Ctx.Dest)= {
   val versionTxt = s"""
     package ammonite
     object Constants{
@@ -305,7 +305,7 @@ def generateConstantsFile(version: String = buildVersion,
 def generateDependenciesFile(scalaVersion: String,
                              fileName: String,
                              deps: Seq[coursier.Dependency])
-                            (implicit ctx: mill.util.Ctx.Dest) = {
+                            (implicit ctx: mill.api.Ctx.Dest) = {
 
   val dir = ctx.dest / "extra-resources"
   val dest = dir / fileName
diff --git a/integration/test/resources/jawn/build.sc b/integration/test/resources/jawn/build.sc
index 8b58efc7..144ef814 100644
--- a/integration/test/resources/jawn/build.sc
+++ b/integration/test/resources/jawn/build.sc
@@ -42,7 +42,7 @@ class JawnModule(crossVersion: String) extends mill.Module{
 
     object play extends Support(){
       def ivyDeps = mill.T{
-        Lib.scalaBinaryVersion(scalaVersion()) match{
+        mill.scalalib.api.Util.scalaBinaryVersion(scalaVersion()) match{
           case "2.10" => Agg(ivy"com.typesafe.play::play-json:2.4.11")
           case "2.11" => Agg(ivy"com.typesafe.play::play-json:2.5.15")
           case _ => Agg(ivy"com.typesafe.play::play-json:2.6.0")
diff --git a/integration/test/resources/play-json/build.sc b/integration/test/resources/play-json/build.sc
index 54e63d65..eb51e999 100644
--- a/integration/test/resources/play-json/build.sc
+++ b/integration/test/resources/play-json/build.sc
@@ -36,7 +36,7 @@ trait PlayJsonModule extends BaseModule with PublishModule with MiMa {
 
   trait Tests extends super.Tests with Scalariform with Headers {
     val specs2Core = T {
-      val v = Lib.scalaBinaryVersion(scalaVersion()) match {
+      val v = mill.scalalib.api.Util.scalaBinaryVersion(scalaVersion()) match {
         case "2.10" => "3.9.1"
         case _ => "4.0.2"
       }
diff --git a/integration/test/resources/play-json/headers.sc b/integration/test/resources/play-json/headers.sc
index 9737cb3d..31613fe6 100644
--- a/integration/test/resources/play-json/headers.sc
+++ b/integration/test/resources/play-json/headers.sc
@@ -2,7 +2,7 @@ import $ivy.`com.github.rockjam::license-headers:0.0.1`
 
 
 import mill._, scalalib._
-import mill.eval.PathRef
+import mill.api.PathRef
 import de.heikoseeberger.sbtheader.{CommentStyle, FileType, HeaderCreator, License}
 
 trait Headers extends ScalaModule {