_project.py: Do not find default targets in .bst directory

Description

In cases where element path is set to . and the project has some junctions, we have to be careful when trying to find the default targets. We can't simply find all files ending with .bst. But, we first need to ensure that we do not recurse down the .bst directory which is used to store BuildStream internals.

Fixes #992 (closed).