Makefile: simplify cockpit repo checkout code

Replace the two rules for test/common and pkg/lib with a single rule
based on `git-archive`, avoiding the use of racy `FETCH_HEAD` and
side-effecting `git-checkout`.  This lets us drop the `flock` call, and
introduces some logic to avoid the fetch in case we already have it.

We'll hardcode shas from here on out: it's more secure (ie:
tamperproof), and more performant (since we don't have to check the ref
if we already have the object), in addition to being easier to work with
in terms of the required git commands.

Use pkg/lib/cockpit.js as a stamp file.

This rule will make it easier to expand to more files in the future.

One drawback is that when we want to update files from the cockpit/
repository, we'll now need to do it in lock-step: using new API from
testlib will force us to adjust to PatternFly changes, for example.

Cherry-picked from cockpit-podman commit cfc05e730d6f5d.

.gitignore

@@ -16,3 +16,4 @@ pkg
 POTFILES*
 tmp/
 /po/LINGUAS
+/tools