EZP-22210: Added specifications for legacy bundles
EZP-22210: Added LegacyBundleInstallCommand.php
EZP-22210: Added legacy extensions locator

Ignored bundles without ezpublish_legacy folder
EZP-22210: Inplemented compiler pass + legacy mapper
EZP-22210: Added LegacyBundles config mapper

The Container parameter ezpublish_legacy.legacy_bundles_extensions,
filled in by the compiler pass, is injected into the mapper
to be injected into legacy.
EZP-22210: Removed LegacyBundleExtension interface
EZP-22210: Changed injection to use merge-settings
EZP-22210: Fixed locator algorithm
EZP-22210: Re-introduced LegacyBundleInterface

Bundles may implement eZ\Bundle\EzPublishLegacyBundle\LegacyBundles\LegacyBundleInterface,
and return an arbitrary list of legacy extensions that must be enabled in the legacy kernel.
EZP-22210: Updated and improved spec