move cargo logic to cargo.py

Summary:
Move cargo logic to separate file as builder.py was getting a bit large

It's just a move of source file for CargoBuilder.  Diff looks a bit big as I did hg cp to preserve history

Differential Revision: D33888925

fbshipit-source-id: 2d57343535ab087e09876edba4d00a6f0234fcd0

build/fbcode_builder/getdeps/manifest.py

@@ -10,7 +10,6 @@ import os
 from .builder import (
     AutoconfBuilder,
     Boost,
-    CargoBuilder,
     CMakeBuilder,
     BistroBuilder,
     Iproute2Builder,
@@ -22,6 +21,7 @@ from .builder import (
     SqliteBuilder,
     CMakeBootStrapBuilder,
 )
+from .cargo import CargoBuilder
 from .expr import parse_expr
 from .fetcher import (
     ArchiveFetcher,