This patch changes
- the extensions.xml generator
- various logic regarding checks if a version exists
to not query the tx_ter_extensions database table anymore.

Now the only place where tx_ter_extensions is actually used
is the SOAP API Upload and the Migration to tx_terfe2_* db structure
which will be migrated in the next patch.

A new Configuration class is added, which contains all information
that was previously set in various places.

This change
* removes a dummy \stdObject containing random information
* removes dependencies revolving around implicit dependencies in TSFE setup variables
* Centralizes all options regarding repository path

In addition, the tx_ter_api object does need any external dependencies
loaded from e.g. pi-based plugins anymore. This way, the next step is
to build a middleware for /ter and removing the plugin alltogether.

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.