cli: FGDB reader: don't load data multiple times when source is a directory

[florianesser]

When using the FileGeodatabase reader from the pro plugins in a command line transformation and the value of the -source parameter is a directory, hale will load the source data from all gdbtables files several times, depending on the number of gdbtables files inside that directory. To prevent that, users need to provide the path to one of the gdbtables files in the -source parameter instead.

Enhance the behaviour in such a way that source data is not loaded multiple times when the source is a directory.