ENH: Add genome_dict functions to GenesDirectoryFormat and ProteinsDirectoryFormat.

[VinzentRisch]

Addition Description
For easier handling of GenesDirectoryFormat and ProteinsDirectoryFormat it would be handy to add a genome_dict function to both of them.

• It will return a dict of IDs as keys and full paths to files as values.
• These functions will work similarly to sample_dict of MultiMAGSequencesDirFmt or feature_dict of MAGSequencesDirFmt.
• GenesDirectoryFormat and ProteinsDirectoryFormat can contain files in per sample directories or not. So depending on the directory structure the output of genome_dict will be similar to sample_dict of MultiMAGSequencesDirFmt or feature_dict of MAGSequencesDirFmt.
• Similar as described in add formats that make sample (or other) ids explicit  bokulich-lab/q2-types-genomics#56
• originally proposed in ENH: add a genome_dict method on all GenomeData-linked formats bokulich-lab/q2-types-genomics#65