Rework SvgSymbolDiscovery to not use Drupal-dependent code

Problem/Motivation

\Drupal\ex_icons\Discovery\SvgSymbolDiscovery has some direct calls to \Drupal. Ideally, we should look at removing this Drupal-specific code. This would then follow the convention of \Drupal\Component\Discovery\YamlDiscovery and \Drupal\Component\Discovery\YamlDirectoryDiscovery that both do not have any Drupal-specific code.

Steps to reproduce

Look at the code.

Proposed resolution

Remove or modify transformFileUrl() from \Drupal\ex_icons\Discovery\SvgSymbolDiscovery. Rework what this method does to somewhere else maybe?

Remaining tasks

TBC

User interface changes

None

API changes

TBC

Data model changes

TBC