pluginhandler: fix stage-snaps for v2 plugins

[jhenstridge]

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The PluginHandler._unpack_stage_snaps method makes use of an attribute on the plugin object that does not seem to exist on v2 plugins, leading to errors like:

...
  File "/snap/snapcraft/5143/lib/python3.6/site-packages/snapcraft/internal/lifecycle/_runner.py", line 247, in _run_pull
    self._run_step(step=steps.PULL, part=part, progress="Pulling")
  File "/snap/snapcraft/5143/lib/python3.6/site-packages/snapcraft/internal/lifecycle/_runner.py", line 322, in _run_step
    self._prepare_step(step=step, part=part)
  File "/snap/snapcraft/5143/lib/python3.6/site-packages/snapcraft/internal/lifecycle/_runner.py", line 313, in _prepare_step
    preparation_function()
  File "/snap/snapcraft/5143/lib/python3.6/site-packages/snapcraft/internal/pluginhandler/__init__.py", line 469, in prepare_pull
    self._unpack_stage_snaps()
  File "/snap/snapcraft/5143/lib/python3.6/site-packages/snapcraft/internal/pluginhandler/__init__.py", line 434, in _unpack_stage_snaps
    logger.debug("Unpacking stage-snaps to {!r}".format(self.plugin.stage_snaps))
AttributeError: 'NilPlugin' object has no attribute 'stage_snaps'

The value being substituted into the log message doesn't really match the text of the log message, so I switched it to use self.part_install_dir instead.

[cjp256]

LGTM, though this indicates we need to expand our stage-snap spread test. I'll do that separately.

Thanks!

[jhenstridge]

I merged @cjp256's spread tests in, but still saw failures. I tried some test builds with some extra debugging, which showed that while the snaps were correctly being unpacked to parts/$name/install, they were subsequently being deleted.

It looks like the culprit was prepare_build(), which clears the install dir when using a v2 plugin. It re-unpacks staged deb packages, but didn't do the same for snaps.