Rebootstrap on latest master (#3420)

Need to make sure everything works and dogfood it a bit before
0.12.0-RC1

This converts Mill over to the `build.mill` file extension. We also
break up `build.mill` into per-folder `package.mill` files:
`scalalib/package.mill`, `main/package.mill`, etc.

IntelliJ users would need go to `Settings` / `Editor` / `File Types` /
`Scala` and add `*.mill`. Github syntax highlighting doesn't work, which
is expected for now until
github-linguist/linguist#7028 lands (which will
need to wait for some usage in the wild according to Github's syntax
highlighting policy)

.config/mill-version

@@ -1 +1 @@
-0.11.12
+0.12.0-M0-68-21fbf2

bsp/package.mill

@@ -0,0 +1,39 @@
+package build.bsp
+// imports
+import mill._
+import mill.contrib.buildinfo.BuildInfo
+import mill.T
+
+
+object `package` extends RootModule with build.MillPublishScalaModule with BuildInfo {
+  def compileModuleDeps = Seq(build.scalalib)
+  def testModuleDeps = super.testModuleDeps ++ compileModuleDeps
+  def buildInfoPackageName = "mill.bsp"
+
+  def buildInfoMembers = T {
+    val workerDep = worker.publishSelfDependency()
+    Seq(
+      BuildInfo.Value(
+        "bsp4jVersion",
+        build.Deps.bsp4j.dep.version,
+        "BSP4j version (BSP Protocol version)."
+      )
+    )
+  }
+
+  override lazy val test: MillScalaTests = new Test {}
+  trait Test extends MillScalaTests {
+    def forkEnv: T[Map[String, String]] = T {
+      // We try to fetch this dependency with coursier in the tests
+      worker.publishLocalCached()
+      super.forkEnv()
+    }
+
+    def forkArgs = super.forkArgs() ++ Seq(s"-DBSP4J_VERSION=${build.Deps.bsp4j.dep.version}")
+  }
+
+  object worker extends build.MillPublishScalaModule {
+    def compileModuleDeps = Seq(build.bsp, build.scalalib, build.testrunner, build.runner) ++ build.scalalib.compileModuleDeps
+    def ivyDeps = Agg(build.Deps.bsp4j, build.Deps.sbtTestInterface)
+  }
+}