Loading pipeline is impossibly slow with > 50 elements

Recent changes to BuildStream have caused load times to go crazy.

For example building core.bst from my Baserock conversion branch takes so long to load the pipeline that I've never seen it finish. However building gnu-toolchain.bst from the same branch (which is a smaller set of elements) works fine.

The problem is in the resolve_project_variant() method -- it seems to touch a combinatorial explosion of the dependency tree.

I reproduced this with commit 97721079 of buildstream.