Avoid depending on 'field_' field name prefix for determining bundle fields

To make it easier to review, I've created a merge request from the patch in #3.

Note that this is a patch to an unsupported branch (8.x-1.x). I haven't yet had a chance to determine if this issue is still present in the 2.x branch, nor if this patch would also apply there.

Accidentally committed this to 8.x-1.x in commit bad9be3 and had to revert it in 2064289.

I've created the merge request correctly this time; sorry for the noise.

Regardless, what I wrote in #7 still applies.

Is this still an issue in the 2.x branch?