Allow configuring an element to not be tracked by --track-all

This might seem weird, but here is the problem. In https://gitlab.gnome.org/GNOME/gnome-build-meta/merge_requests/18 @abderrahimk says that the recommended workflow for GNOME developers using BuildStream should now be:

bst track base/freedesktop-sdk-junction.bst
bst track --deps all --except base.bst core/your-module.bst
bst build core/your-module.bst

or, a bit simpler:

bst track base/freedesktop-sdk-junction.bst
bst build --track-all --track-except base.bst core/your-module.bst

But this is really too hard, now. We should try to keep things as simple as possible. Previously, we had a nice workflow where bst build core/your-module.bst was equivalent to jhbuild buildone, and bst build --track-all core/your-module.bst was equivalent to jhbuild build. Having to do track separately would be OK, but having to remember extra args like --deps all --except base.bst is altogether too hard. Similarly, in the one-line version, --track-except base.bst adds too much typing.

One solution would be to somehow configure the project such that base.bst is always skipped when using --track-all or track --deps all, such that the only way to track it would be to run bst track base.bst. Maybe there are other solutions. Thoughts?