New parser libraries are not available unless uninstall and re-install module

Problem/Motivation

When following the readme or documentation pages to add new parser libraries, the library is not available unless the markdown module is uninstalled and re-installed.

I think this could be as designed given the logic found in markdown_requirements(), but I'm not entirely sure.

Steps to reproduce

• Enable module
• Add new markdown library via composer, e.g. composer require michelf/php-markdown:^2.0
• Go to /admin/config/content/markdown
• Observe parser (e.g. PHP Markdown) is listed as unavailable
• Expected behavior: parser is listed as available

Proposed resolution

TBD

Remaining tasks

• Identify the root cause
• Consider options
• Choose options

User interface changes

TBD

API changes

TBD

Data model changes

TBD