The database table tx_ter_extensionkeys containing all registered
extensionkeys is dropped in favor of "tx_terfe2_domain_model_extension".

This table was synced already anyways, and can now be used within extensions.typo3.org
without having to sync with a cronjob anymore.

Breaking: The information about "title" and "description" is removed, as it wasn't
required via the Web GUI when registering a key already.

The sync task / TER importer now only imports the uploaded versions, the SOAP API
now creates records directly in tx_terfe2_domain_model_extension when
registering a new extension key.

The One-Time-Migration-Script by tomalo (ImportAllExtensionKeysTask) is
now removed as it is not needed anymore.

There is a simple connection between tx_ter_extensiondetails and tx_ter_extensions
(extensionuid field), but both tables always need to be kept in sync. This is unnecessary
overhead. For this reason, all data is now moved into tx_ter_extensions (which will
later be moved into tx_terfe2_domain_model_version).

After adding the new fields to the database table "tx_ter_extensions",
A onetime CLI command "ter:migrateextensiondetails" is executed
and syncs all existing data into the main "tx_ter_extensions".

"tx_ter_extensiondetails" can be dropped after the migration, as it is not
used anymore at all.

This change re-implements the generation of the XML file for TYPO3 installations
by moving the relevant code from EXT:ter to ter_fe2.

CombinedExtensionRepository -> does the collection of available extensions
ExtensionIndexService -> handles the XML generation and writing to files

This way, all functionality can be separated and exchanged if necessary.

A CLI command "ter:createExtensionIndexXml" is added which
also shows some more useful output, however the logic is still in
the "UpdateCurrentVersionListTask" class in EXT:ter.

The methods:
tx_ter_helper->requestUpdateOfExtensionIndexFile
tx_ter_helper->writeExtensionIndexfile

are removed, and thus, all of this logic is migrated to Doctrine DBAL.

Root composer.json is adapted so the necessary PHP extensions are available
in your IDE.