I think I understand now, courtesy of this post. It seems that Tkinter is set aside from the others (due to being much larger?) on purpose. I have to include it as a stage-package.
With that said, his solution no longer seems to work for me:
files: #to deal with problem finding init.tcl
plugin: dump
source: /usr/share/tcltk/
organize:
'*': ./usr/lib/
I get the following error message:
Failed to pull source: unable to determine source type of '/usr/share/tcltk'.
Check that the URL is correct or consider specifying `source-type` for this part. See `snapcraft help sources` for more information.
Are absolute directories not allowed anymore? If not, what should my workaround be?