There is some heavy legacy code regarding "where to find details/ the extension files"
exactly in TER FE2.

* Mirror -> use a mirror from extension repository (none given in TER), 5 tx_terfe2_domain_model_version entries
* SOAP -> fetch the t3x from the SOAP API (no tx_terfe2_domain_model_version entry)
* File -> default for all other files

The type for each extension version is stored in "tx_terfe2_domain_model_version.extension_provider" and is removed
to reduce massive code and complexity, having only the "FileProvider" in place
removing the ProviderManager, Interface etc.

MirrorService, MirrorPovider, SoapProvider, SoapService can be removed
reducing cross-concern functionality.