Move `util-appmacro` to sbt as `core-macros`

This commit moves everything behind the old util-appmacro over to
sbt/sbt.

appmacro has been renamed as core-macros. It is indeed the project
that defines the main logic to manipulate macros, and defines interfaces
that are then implemented by main-settings. Because of this strong
relationship between these two modules, I find that they have to stay
together and be able to be modified in one commit. I've realised this
while hacking on the macro changes for the usability of sbt.

The reason for this change is because the logic of this module is really
tight to the implementation of sbt and, I would even say, essentially
define the sbt DSL.

build.sbt

@@ -249,6 +249,16 @@ lazy val commandProj = (project in file("main-command"))
              addSbtCompilerClasspath,
              addSbtLm)
 
+// The core macro project defines the main logic of the DSL, abstracted
+// away from several sbt implementators (tasks, settings, et cetera).
+lazy val coreMacrosProj = (project in file("core-macros")).
+  settings(
+    commonSettings,
+    name := "Core Macros",
+    libraryDependencies += "org.scala-lang" % "scala-compiler" % scalaVersion.value
+  )
+    .configure(addSbtUtilCollection)
+
 // Fixes scope=Scope for Setting (core defined in collectionProj) to define the settings system used in build definitions
 lazy val mainSettingsProj = (project in file("main-settings"))
   .dependsOn(commandProj, stdTaskProj)