devel/py-platformdirs: Unbreak after 2.5.2 update

The upgrade of py-platformdirs to 2.5.2 breaks pylint (and most
certainly anything else that uses the module):

lns 57 % pylint [...] lblcache.py
[...]
ModuleNotFoundError: No module named 'platformdirs'

Apply the tweak described here to adjust for the "src" subdir in
the distribution:

https://stackoverflow.com/a/36012825/2994620

PR: 263639
Approved by: koobs (portmgr blanket: runtime bugfix)