Add a command for a dirty, noncompiling scala repl.

For debugging purposes, it's often useful to be able to open a repl
on the existing binaries, without having to get code into a currently
compilable state. This provides an alternative scala repl command,
"repl_dirty" that doesn't have the compile dependency. If pants generated binaries
for the targets don't exist, this will fail with an error.

(sapling split of fa19f01e5b14e469852de04ed31a7afee5904051)

src/python/twitter/pants/commands/goal.py

@@ -608,6 +608,13 @@ def is_java(target):
 ).install('repl').with_description(
   'Run a (currently Scala only) REPL with the classpath set according to the targets.')
 
+goal(
+  name='scala-repl-dirty',
+  action=ScalaRepl
+).install('repl-dirty').with_description(
+  'Run a (currently Scala only) REPL with the classpath set according to the targets, \n' +
+    'using the currently existing binaries, skipping compilation')
+
 goal(
   name='filedeps',
   action=FileDeps