python plugin ignores source-subdir

[Leo Arias]

When the setup.py file is in a subdir of the project, the python plugin can't find it and thus finishes without installing anything.

To reproduce, let's modify the python-hello test snap:

$ mkdir tmp
$ cd tmp
$ git clone https://github.com/snapcore/snapcraft
$ cp -R snapcraft/snapcraft/tests/integration/snaps/python-hello/ .
$ cd python-hello
$ mkdir subdir
$ mv hello subdir/
$ mv setup.py subdir/
$ echo " source-subdir: subdir" >> snap/snapcraft.yaml
$ snapcraft

[Leo Arias (elopio)]

The python plugin refers always to self.sourcedir. It should instead refer to self.builddir, which the base plugin updates to point to source-subdir if present.

[Leo Arias (elopio)]

This affects the sawtooth snap: https://github.com/elopio/sawtooth-core/blob/snapcraft/snap/snapcraft.yaml