The `setup.py` parse-info method treats description/long_description keys wrongly

[林博仁(Buo-ren, Lin)]

According to the [setuptools documentation](https://setuptools.readthedocs.io/en/latest/setuptools.html?highlight=long_description#id54) the `description` key is equivalent to Snapcraft's `summary` key and the `long_description` key is equivalent to Snapcraft's `description` key, however currently Snapcraft uses the `description` key's value as the snap description and ignores the `long_description` key in setup.py, I would like to request changing the behavior to match with setuptools.

[林博仁(Buo-ren, Lin) (buo-ren-lin)]

https://github.com/snapcore/snapcraft/pull/2518

[Merlijn Sebrechts (merlijn-sebrechts)]

`long_description` isn't actually a good candidate for the `description` field in snapcraft because the text is actually used for the project homepage on pypi (and often also on Github).

As a result, many applications start `long_description` by explaining how to install the software on Windows, Mac an Linux. It's also often in an incompatible format and too long for the `description` field.

More info: https://github.com/snapcore/snapcraft/pull/2756#issuecomment-544284814