Source distribution cannot include symlinks

Reporting as minor as I don't know if this is really a problem with BuildStream's CI setup or with setuptools.

!295 (merged) added a test which used an import element of a directory which contained a symlink. However, setup.py sdist will not capture that file, giving this warning, easily missed:

'tests/integration/project/files/compose-symlinks/base/sbin' not a regular file -- skipping

Since all our CI tests create a source distribution and test that we ended up with an unfortunate situation where tests passed locally but not on CI. Direct import of a directory (not a tarball or zip) containing a symlink seems like something we should test but we can't at the moment. There don't appear to be any options to sdist to include symlinks in the archive.

I can't think of a clean solution, but work-arounds are:

1. Regenerate the symlink in the compose.py test script
2. Add another stage to CI which runs tests from a git checkout instead of the source dist