Rename $module.info.yml into extension.yml

Follow-up to: #1793074: Convert .info files to YAML

Problem

• D8 requires a one extension per directory rule since #1299424: Allow one module per directory and move system tests to core/modules/system.
• This rule is not enforced in any way.

Goal

• Enforce one extension per directory by using a static .info file name.

Proposed solution

1. Rename all $module.info.yml files into extension.yml

Examples

Notes

• extension.yml is meant literally; i.e., "extension" is NOT to be replaced with the extension's name.
• This inherently prevents two extensions to live in the same directory.
• That's a critical aspect of D8's new architecture, but it is not enforced anywhere.
• Therefore, an extension.yml (or perhaps even composer.yml?) file for each extension inherently enforces the architectural requirement that is not enforced in any way yet.
• intuitive++