devserver: Add support for control_files.tar and autotest_packages.tar

We are splitting up the autotest.tar tarball into 2 separate tarballs.
This change updates the devserver code so that it is able to properly
stage these artifacts.

BUG=chromium:421122
TEST=local devserver and local moblab build that tested a trybot image
with no autotest.tar artifact.

Change-Id: I546add63b74906ce2969e45f8209e074bed02b5c
Reviewed-on: https://chromium-review.googlesource.com/226822
Reviewed-by: Prashanth B <beeps@chromium.org>
Reviewed-by: Yu-Ju Hong <yjhong@chromium.org>
Commit-Queue: Simran Basi <sbasi@chromium.org>
Tested-by: Simran Basi <sbasi@chromium.org>
diff --git a/artifact_info.py b/artifact_info.py
index 89fbf3e..0a5c674 100644
--- a/artifact_info.py
+++ b/artifact_info.py
@@ -40,6 +40,12 @@
 # Autotest -- the main autotest directory without the test_suites subdir.
 AUTOTEST = 'autotest'
 
+# Control Files -- the autotest control files without the test_suites subdir.
+CONTROL_FILES = 'control_files'
+
+# Autotest Packages-- the autotest packages subdirectory.
+AUTOTEST_PACKAGES = 'autotest_packages'
+
 # Test Suites - just the test suites control files from the autotest directory.
 TEST_SUITES = 'test_suites'