[5.3] Add PluginHelper::getPlugins() and deprecate old behavior

[Hackwar]

Pull Request for Issue #44270.

Summary of Changes

PluginHelper::getPlugin() currently returns either an (empty) array of objects or a single object. This is the first step towards cleaning that code up by splitting the code up into a method to retrieve a single plugin (getPlugin()) and an array of plugins (getPlugins()).

Testing Instructions

Codereview.

Link to documentations

Please select:

• [ ] Documentation link for docs.joomla.org:

• [ ] No documentation changes for docs.joomla.org needed

• [ ] Pull Request link for manual.joomla.org:

• [ ] No documentation changes for manual.joomla.org needed

[brbrbr]

This pull request does not solve the issue of:

Warning: Attempt to read property "params" on array in [ROOT]libraries/src/Component/Router/Rules/MenuRules.php

The sef plugin must be enabled, always.

[ghost]

This pull request does not solve the issue of:

Warning: Attempt to read property "params" on array in [ROOT]libraries/src/Component/Router/Rules/MenuRules.php

@brbrbr Test #44283

[laoneo]

When I see this code, then the plugin should either be booted or the PluginsModel should be used instead of introducing another static function with deprecated global static Factory calls.

[HLeithner]

This pull request has been automatically rebased to 6.0-dev.

[HLeithner]

This pull request has been automatically rebased to 6.1-dev.