Same default filename for multiple MF6 package instances

[aprovost-usgs]

When there is more than one of the same kind of stress package (say, two or more WEL packages) in an MF6 model, the default filename assigned by flopy is the same for all instances of the package, creating a conflict when MF6 tries to open more than one file with the same name. This can be worked around by explicitly assigning different filenames to the packages using the filename keyword argument. (Observed using flopy 3.6.6 but likely pertains to previous versions, as well.)

[spaulins-usgs]

@aprovost-usgs, this should be fixed with the latest PR.