Return a dictionary with descriptions keyed on source package names.
Takes an ISelectResults of a *PackageName query. The use_names
flag is a hack that allows this method to work for the
BinaryAndSourcePackageName view, which lacks IDs.
WARNING: this function assumes that there is little overlap and much
coherence in how package names are used, in particular across
distributions if derivation is implemented. IOW, it does not make a
promise to provide The Correct Description, but a pretty good guess
at what the description should be.