Document and test workflow for aliasing external modules (#3760)

Fixes #299

I'm a bit surprised that this just works, but it does! 

The module's `millSourcePath` and `out/` folder path remain at their
original locations as the `mill.define.Ctx` is still provided by the
top-level `ExternalModule` constructor, it's just during resolution the
added method `def` allows it to get picked up during object graph
traversal

docs/modules/ROOT/pages/fundamentals/modules.adoc

@@ -58,3 +58,7 @@ that is shared by the entire build: for example,
 service & cache that is shared between all ``ScalaModule``s, and
 `mill.scalalib.GenIdea/idea` lets you generate IntelliJ projects without
 needing to define your own `Task.Command` in your `build.mill` file
+
+== Aliasing External Modules
+
+include::partial$example/fundamentals/modules/10-external-module-aliases.adoc[]

example/fundamentals/modules/10-external-module-aliases/build.mill

@@ -0,0 +1,27 @@
+// Mill allows you to alias external modules via `def`. You can use this to add
+// shorthand aliases for external modules that have long names, such as
+// `mill.javalib.palantirformat.PalantirFormatModule` below:
+
+package build
+import mill._, javalib._
+
+object foo extends JavaModule
+
+def myAutoformat = mill.javalib.palantirformat.PalantirFormatModule
+
+/** Usage
+
+> cat src/foo/Foo.java # starts off unformatted
+package foo;public class Foo{ public static void main(String[] args) {System.out.println("Hello World!");}}
+
+> mill myAutoformat # easier to type than `./mill mill.javalib.palantirformat.PalantirFormatModule/`
+
+> cat src/foo/Foo.java # code is now formatted
+package foo;
+public class Foo {
+  public static void main(String[] args) {
+    System.out.println("Hello World!");
+  }
+}
+
+*/

example/fundamentals/modules/10-external-module-aliases/foo/src/foo/Foo.java

@@ -0,0 +1 @@
+package foo;public class Foo{ public static void main(String[] args) {System.out.println("Hello World!");}}