Changes how to calculate sources and bases in spindle

Calculate the source roots for all spindle targets in the graph rather than just the invalid ones.

Testing Done:
All Travis CI tests are green.

Bugs closed: 1603

Reviewed at https://rbcommons.com/s/twitter/r/2282/

contrib/spindle/src/python/pants/contrib/spindle/tasks/spindle_gen.py

@@ -21,7 +21,6 @@
 
 from pants.contrib.spindle.targets.spindle_thrift_library import SpindleThriftLibrary
 
-
 class SpindleGen(NailgunTask):
   def __init__(self, context, workdir):
     super(SpindleGen, self).__init__(context, workdir)
@@ -84,7 +83,11 @@ def sources_generated_by_target(self, target):
     ]
 
   def execute_codegen(self, targets):
-    bases, sources = self._calculate_sources(targets, lambda t: isinstance(t, SpindleThriftLibrary))
+    sources = self._calculate_sources(targets, lambda t: isinstance(t, SpindleThriftLibrary))
+    bases = set(
+      target.target_base
+      for target in self.context.targets(lambda t: isinstance(t, SpindleThriftLibrary))
+    )
     scalate_workdir = os.path.join(self.workdir, 'scalate_workdir')
     safe_mkdir(self.namespace_out)
     safe_mkdir(scalate_workdir)
@@ -207,16 +210,13 @@ def execute(self):
         self.update_artifact_cache(vts_artifactfiles_pairs.items())
 
   def _calculate_sources(self, thrift_targets, target_filter):
-    bases = set()
     sources = set()
     def collect_sources(target):
       if target_filter(target):
-        bases.add(target.target_base)
         sources.update(target.sources_relative_to_buildroot())
-
     for target in thrift_targets:
       target.walk(collect_sources)
-    return bases, sources
+    return sources
 
 
 # Slightly hacky way to figure out which files get generated from a particular thrift source.