_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).