[PP-3] Deprecate dependencies, type and version in *info.yml files and use composer.json instead

Problem/Motivation

Issues we're trying to solve

• core: 8.x in an info file is restrictive
• we invented our own dependency syntax that is overly complex because of the optional core major version
• we want to move away from having the major version in the module version (e.g. 8.x-1.x » 1.0.0)
• we want to move core away from version: VERSION so we can do subtree splitting
• we want contrib modules to be able to declare their composer dependencies - e.g my module 1.3 needs symfony/yaml 4.0
• as a bonus, themes could rely on modules

@sun suggested this in #1936886-21: Rename $module.info.yml into extension.yml

Proposed resolution

use composer.json for

• discovery of themes/modules/profiles/engines
• dependency declaration
• version numbering

Deprecate the dependencies, type and version keys in info.yml files

Remaining tasks

Wind back to the scope of comment #39

User interface changes

API changes

Data model changes