Have downloader rsync everything at once for speed

This speeds up the copy *significantly*.  The extra SSH calls were
adding up to a lot of extra cost.  3 seconds for a NOP download!

Do this by building up a folder that looks like what the target should
look like, and then rsync that all in 1 go.

While we are here, rename .stripped to not have the extension.  It makes
the commands harder to remember.

Change-Id: I50e50c90421e049f48af453c8113d7c2204c7774
Signed-off-by: Austin Schuh <austin.linux@gmail.com>
diff --git a/frc971/downloader/downloader.bzl b/frc971/downloader/downloader.bzl
index e1b535e..9eb7e32 100644
--- a/frc971/downloader/downloader.bzl
+++ b/frc971/downloader/downloader.bzl
@@ -1,5 +1,12 @@
 def _aos_downloader_impl(ctx):
     all_files = ctx.files.srcs + ctx.files.start_srcs + [ctx.outputs._startlist]
+    target_files = []
+
+    # downloader looks for : in the inputs and uses the part after the : as
+    # the directory to copy to.
+    for d in ctx.attr.dirs:
+        target_files += [src.short_path + ":" + d.downloader_dir for src in d.downloader_srcs]
+
     ctx.actions.write(
         output = ctx.outputs.executable,
         is_executable = True,
@@ -7,16 +14,12 @@
             "#!/bin/bash",
             "set -e",
             'cd "${BASH_SOURCE[0]}.runfiles/%s"' % ctx.workspace_name,
-        ] + ['%s --dir %s --target "$@" --type %s %s' % (
-            ctx.executable._downloader.short_path,
-            d.downloader_dir,
-            ctx.attr.target_type,
-            " ".join([src.short_path for src in d.downloader_srcs]),
-        ) for d in ctx.attr.dirs] + [
-            'exec %s --target "$@" --type %s %s' % (
+        ] + [
+            'exec %s --target "$@" --type %s %s %s' % (
                 ctx.executable._downloader.short_path,
                 ctx.attr.target_type,
                 " ".join([src.short_path for src in all_files]),
+                " ".join(target_files),
             ),
         ]),
     )