bst build does not handle elements with Consistency.INCONSISTENT

When typing bst build with inconsistent sources (sources that have no ref and need to be tracked), BuildStream just goes ahead and starts to try building these, but will inevitably fail because there are no refs.

We could have different approaches here:

• Just bail out with a message to the user
• Try building any elements which could possibly be built, but omit any inconsistent elements (by prefiltering the element list)