add a path argument to the stage command

Allow users to use spack to stage a, potentially complex, package into a
given path.  This is nice for packages with multiple resources that must
be placed, for example LLVM with all sub-projects.

lib/spack/spack/cmd/stage.py

@@ -35,6 +35,9 @@ def setup_parser(subparser):
     subparser.add_argument(
         '-n', '--no-checksum', action='store_true', dest='no_checksum',
         help="Do not check downloaded packages against checksum")
+    subparser.add_argument(
+        '-p', '--path', dest='path',
+        help="Path to stage package, does not add to spack tree")
 
     subparser.add_argument(
         'specs', nargs=argparse.REMAINDER, help="specs of packages to stage")
@@ -50,4 +53,7 @@ def stage(parser, args):
     specs = spack.cmd.parse_specs(args.specs, concretize=True)
     for spec in specs:
         package = spack.repo.get(spec)
-        package.do_stage()
+        if args.path:
+            package.do_stage(path=args.path)
+        else:
+            package.do_stage()

lib/spack/spack/package.py

@@ -709,14 +709,18 @@ def do_fetch(self, mirror_only=False):
         if spack.do_checksum and self.version in self.versions:
             self.stage.check()
 
-
-    def do_stage(self, mirror_only=False):
+    def do_stage(self, mirror_only=False, path=None):
         """Unpacks the fetched tarball, then changes into the expanded tarball
            directory."""
+
         if not self.spec.concrete:
             raise ValueError("Can only stage concrete packages.")
 
         self.do_fetch(mirror_only)
+
+        if path is not None:
+            self.stage.path = path
+
         self.stage.expand_archive()
         self.stage.chdir_to_source()