Fix snapcraft.yaml to properly use stage-packages

[dilyn-corner]

Expanding the PYTHONPATH is required to pickup the germinate stage-package

Without specifying PTYHONPATH you may hit such an error:

[8] germinate
Traceback (most recent call last):
  File "/snap/ubuntu-image/512/usr/bin/germinate", line 25, in <module>
    from germinate.scripts.germinate_main import main
ModuleNotFoundError: No module named 'germinate'

Additionally, debootstrap is a stage-package but will not be used -- if debootstrap is not installed on the host, you'll run into both a "missing from $PATH" error and, after you correct the $PATH variable used in the snapcraft.yaml, you end up with debootstrap being unable to find some relevant files in /usr/share -- this can be resolved by setting an environment variable DEBOOTSTRAP_DIR.

The last commit in this PR adds two new stage-packages: devscripts and grub-common. It's possible that a user will not have these packages installed, and they are requirements for building some variations of our pc-gadget snap (https://github.com/snapcore/pc-gadget/tree/classic). I leave it up to whomever reviews this to determine if these are valid inclusions for ubuntu-image.

[codecov]

Codecov Report

Merging #137 (2b663a1) into main (28b7b85) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #137   +/-   ##
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  Coverage   88.71%   88.71%           
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  Files          11       11           
  Lines        2383     2383           
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  Hits         2114     2114           
  Misses        265      265           
  Partials        4        4           

[sil2100]

Thank you for this PR @dilyn-corner ! The changes themselves look good, I just need to run some tests to see if everything is still working as expected. The additional stage-packages feel okay as well - we wanted to leave those out and leave this to gadget owners to recommend packages to install, but I think they're common enough in the gadget-build world that they make sense.
Would be nice if we had an interface for the gadget tree to inform ubuntu-image and the user of what build dependencies it requires. But that'll be made obsolete with imagecraft.

[dilyn-corner]

Yes I was very hesitant on adding the requirements for building some gadgets -- this would be less of an issue if ubuntu-image would use snapcraft to build the gadget instead of make, but this is a whole other debacle :)

I'm certain my testing isn't as comprehensive as yours, but I've been building a few dozen different (Classic) images over the last few days with these changes and haven't noticed any breaks FWIW.

[sil2100]

Looks good indeed!

As for using snapcraft for building the gadgets - we actually long time ago initially used it indeed! Then had to change to Makefiles due to snapcraft being in Ubuntu universe. Now we could revisit this eventually, but I think all of these problems will go away when we switch to imagecraft.