- rename dependency to library
- drop load_order from libraries table
- add python_versions to library_releases table

- add migration scheme file

commit aede06d414bb9a3a6f710690f2f3f698c7303beb

Scheme 4.0.0 is incompatible with 3.0.0 and will cause current PC 3.4.x
releases to completely fail. They wouldn't even be able to fetch data
to upgrade itself.

Hence a dummy channel_v3.json needs to be served for all clients not
updated before new packagecontrol.io release goes online.

PC 4.0 can ship with both channel_v3 and _v4 pre-configured to ensure a
smooth transition to new scheme version.

The only thing changing for end users is availability of libraries for
python 3.8 then.

This change caused explicit data from JSON to be overwritten with
fallback defaults.

see: wbond/package_control@a8262a9

__all__ is a list of names (strings) of exported modules.

Fixes wbond#158

source: wbond/package_control@32c0e0b

source: wbond/package_control@4edd2a7

This commit enables packagecontrol.io to optionally store packages'
python_versions, which are declared within a repository.json

This way old packages, which are proofed compatible with python 3.8, can
be pushed to new plugin_host without touching their github repo.

A local .python-version file always overrides upstream settings.

Provide fallbacks if release keys are not found. Primarily possible only for
`python_versions` as it is optional for packages.

Adds missing python_versions column to package releases.

This commit

1. moves asset storage functionality to a dedicated module as it might also be
   used to provide compressed stylesheets or javascript assets in future.

2. fixes an issue which caused generated channel.json files to look different
   after each poll. This causes the file to change each 10mins even if content
   hasn't changed. This degrades chance to return 304 so clients could re-use
   local cache.

   To achieve it, json objects content is sorted by key and a hash file is used
   to check, whether content has changed since last run.