Fix project.properties dependency issue

Fix project.properties dependency issue

Correctly place scrlib references also
in the case there is no project.properties file
present in the original repository.

This commit fixes a problem, which was introduced with
021df3c42420de542bf11f22948831023b47ebf3: If no
project.properties file exists in the original
project's source repo, place_srclib will not place
a dependency --- even if there should be one:

>    if not os.path.isfile(proppath):
>        return

Example app: fdroiddata/metadata/org.berlin_vegan.bvapp.txt