BUG when building a project with junctions in non-strict mode

Summary

A stack trace when loading the project with a junction in non-strict mode.

Steps to reproduce

Use gnome-build-meta branch juerg/strip-variables (or any project with junction), and try bst --no-strict build core.bst.

What is the current bug behavior?

I get the following stack trace:

BUG     join() argument must be str or bytes, not 'NoneType'

    Traceback (most recent call last):
      File "/home/abderrahim/.virtualenvs/bst-master/bin/bst", line 8, in <module>
        sys.exit(cli())
      File "/home/abderrahim/.virtualenvs/bst-master/lib/python3.7/site-packages/click/core.py", line 764, in __call__
        return self.main(*args, **kwargs)
      File "/home/abderrahim/dev/bst-master/buildstream/_frontend/cli.py", line 195, in override_main
        standalone_mode=standalone_mode, **extra)
      File "/home/abderrahim/.virtualenvs/bst-master/lib/python3.7/site-packages/click/core.py", line 717, in main
        rv = self.invoke(ctx)
      File "/home/abderrahim/.virtualenvs/bst-master/lib/python3.7/site-packages/click/core.py", line 1137, in invoke
        return _process_result(sub_ctx.command.invoke(sub_ctx))
      File "/home/abderrahim/.virtualenvs/bst-master/lib/python3.7/site-packages/click/core.py", line 956, in invoke
        return ctx.invoke(self.callback, **ctx.params)
      File "/home/abderrahim/.virtualenvs/bst-master/lib/python3.7/site-packages/click/core.py", line 555, in invoke
        return callback(*args, **kwargs)
      File "/home/abderrahim/.virtualenvs/bst-master/lib/python3.7/site-packages/click/decorators.py", line 27, in new_func
        return f(get_current_context().obj, *args, **kwargs)
      File "/home/abderrahim/dev/bst-master/buildstream/_frontend/cli.py", line 387, in build
        remote=remote)
      File "/home/abderrahim/dev/bst-master/buildstream/_stream.py", line 242, in build
        dynamic_plan=True)
      File "/home/abderrahim/dev/bst-master/buildstream/utils.py", line 1303, in _gc_disabled
        return func(*args, **kwargs)
      File "/home/abderrahim/dev/bst-master/buildstream/_stream.py", line 1041, in _load
        fetch_subprojects=fetch_subprojects)
      File "/home/abderrahim/dev/bst-master/buildstream/_pipeline.py", line 113, in load
        fetch_subprojects=fetch_subprojects)
      File "/home/abderrahim/dev/bst-master/buildstream/_project.py", line 456, in load_elements
        fetch_subprojects=fetch_subprojects)
      File "/home/abderrahim/dev/bst-master/buildstream/_loader/loader.py", line 114, in load
        element = loader._load_file(name, rewritable, ticker, fetch_subprojects)
      File "/home/abderrahim/dev/bst-master/buildstream/_loader/loader.py", line 263, in _load_file
        fetch_subprojects, dep.provenance)
      File "/home/abderrahim/dev/bst-master/buildstream/_loader/loader.py", line 263, in _load_file
        fetch_subprojects, dep.provenance)
      File "/home/abderrahim/dev/bst-master/buildstream/_loader/loader.py", line 263, in _load_file
        fetch_subprojects, dep.provenance)
      File "/home/abderrahim/dev/bst-master/buildstream/_loader/loader.py", line 258, in _load_file
        fetch_subprojects=fetch_subprojects, provenance=dep.provenance)
      File "/home/abderrahim/dev/bst-master/buildstream/_loader/loader.py", line 559, in _get_loader
        filename, element._get_cache_key())
      File "/home/abderrahim/.virtualenvs/bst-master/lib/python3.7/posixpath.py", line 94, in join
        genericpath._check_arg_types('join', a, *p)
      File "/home/abderrahim/.virtualenvs/bst-master/lib/python3.7/genericpath.py", line 149, in _check_arg_types
        (funcname, s.__class__.__name__)) from None
    TypeError: join() argument must be str or bytes, not 'NoneType'

Possible fixes

Bisecting this gives it started happening in 352f4ad9. It seems element._get_cache_key() is returning None for some reason.

Other relevant information

• BuildStream version affected: current master 887461a5

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