[engine] model snapshots in validation, make root rules a dict instea…

…d of a set (pantsbuild#4125)

### Problem
I'm working on porting my patch for pantsbuild#3580 to the native engine, (pantsbuild#3960). First, I'm updating the graph construction code with the changes there. It hadn't covered Snapshot processes, and it modeled root rules as a set rather than a graph, which caused problems.

### Solution

Include snapshot process' behavior in the rule graph construction used by the validator. Update tests to expect the new visualization format.

### List of changes
* rename NodeBuilder to RuleIndex
* rename RootRule to RootRuleGraphEntry
* support SnapshotProcess in validator
* Name snapshot process funcs so they are printable
* Include subject and product types in intrinsic rule repr
* Include graph generation methods that can construct new graphs from an existing one--these are currently unused
* Raise an error if a declared rule is not reachable from the declared root types.
* Unify naming for input selector for projection selectors
* If field of a SelectDependencies is the default value, don't include it in the repr
* Include a map of root rule to the rules that would fulfill it.

src/python/pants/engine/isolated_process.py

@@ -199,7 +199,9 @@ def ptree(func):
 
 def create_snapshot_intrinsics(project_tree):
   def ptree(func):
-    return functools.partial(func, project_tree, snapshot_directory(project_tree))
+    partial = functools.partial(func, project_tree, snapshot_directory(project_tree))
+    partial.__name__ = '{}_intrinsic'.format(func.__name__)
+    return partial
   return [
       (Snapshot, Files, ptree(create_snapshot_archive)),
     ]
@@ -213,7 +215,9 @@ def create_snapshot_tasks(project_tree):
   uncacheable singleton.
   """
   def ptree(func):
-    return functools.partial(func, project_tree, snapshot_directory(project_tree))
+    partial = functools.partial(func, project_tree, snapshot_directory(project_tree))
+    partial.__name__ = '{}_task'.format(func.__name__)
+    return partial
   return [
       (Snapshot, [Select(Files)], ptree(create_snapshot_archive)),
     ]