dependencies.yaml: allow dependency to module without dependencies.yaml

Allow product modules to define dependecies.yaml with modules that do not have dependencies.yaml themselves. This allow products to depend to e.g.,  qtbase < 6.0.0, which does not use dependencies.yaml.

current such dependencies fail with:

Creation of work items failed: Error reading 'dependencies.yaml' in 'qt/qtbase' (abfb1b8665923ce2824392f3a04e5e4ac3871017): Check that repository, sha1 and path are valid.