This is equivalent to the tar source, but for Zip archives.

The utility function was added in 08da7cc7, but it never was removed
from here.

This test was encoding tarballs with '.', so the change of
ignoring '.' breaks this - now we just make the tar test
scaffolding use an empty string for the base-dir.

Attempting to compose a list property would result in an unhandled
exception:

    AttributeError: 'CommentedSeq' object has no attribute 'get'

We now at least detect the situation and produce an exception that
the frontend will report neatly:

    Error loading pipeline: element.bst [line 11 column 4]: Only values of type 'dict' can be composed.

I was getting this error from an attempt to conditionally extend the
sources list:

    sources:
      (?):
        arch == "x86_64":
          - url: http://example.com/x86_64

The correct way to do this is to move the conditional into the parent
dict, e.g.:

    (?):
      arch == "x86_64":
        sources:
          - url: https://example.com/x86_64

It would be nice if the error message could hint at how the user can do
what they want, but it doesn't seem possible in this case.

This wont take any additional time because if there is
a remote artifact share configured in the pytest suite, it
can only be a locally created one - this makes the push/pull
tests work after recently making downloading of remote refs
optional for the sake of issue #140.

First case simply asserts that prepending to configure-commands
from the project.conf works as intended, ensuring we dont regress
issue #127

When using an undefined option in a conditional expression, buildstream
should error out and print a good message about it.