Import elements allow build dependencies

Import elements currently allow build dependencies even though they are never used, except for scheduling/pipeline purposes. This may be confusing to users.

We should set BST_FORBID_BDEPENDS in import.py. However, various test cases currently rely on import elements supporting build dependencies. While some of them should be easy to fix (change depends to runtime-depends), others might indeed need the build dependencies as they test scheduler/pipeline behavior. Thus, test case changes need to be done very carefully.

I've pushed the juerg/import-no-build-deps branch as a possible starting point.

However, various test cases currently rely on import elements supporting build dependencies.

Good point, just pointing out that we also need to keep track of this breaking change for a porting guide.

I expect that people have been using "depends:" on import elements for the purpose of enforcing staging order, and probably did not bother to specify that it is a runtime-only dependency.