Allow builds to be started with an external archive URL

Fixes #114

buildman/jobutil/buildjob.py

@@ -62,6 +62,17 @@ class BuildJob(object):
   def repo_build(self):
     return self._load_repo_build()
 
+  def get_build_package_url(self, user_files):
+    """ Returns the URL of the build package for this build, if any or empty string if none. """
+    archive_url = self.build_config.get('archive_url', None)
+    if archive_url:
+      return archive_url
+
+    if not self.repo_build.resource_key:
+      return ''
+
+    return user_files.get_file_url(self.repo_build.resource_key, requires_cors=False)
+
   @property
   def pull_credentials(self):
     """ Returns the pull credentials for this job, or None if none. """