[BUGFIX] Fixed current extensionscope for fetching viewconfiguration

FluxService::getViewConfigurationForExtensionName() should fetch
TypoScriptSubConfiguration using the extension key, not extension
names like "Vendor.ExtName"
Merge pull request #598 from higidi/bugfix/extensionscope-for-fetching-viewconfiguration

[BUGFIX] Fixed current extensionscope for fetching viewconfiguration
[TASK] Accomodate 6.3 branches in unit test
[BUGFIX] Ensure argument types in BackendConfigurationManager

Also implemented an intermediate variable to avoid a repeated function call.

Close: #596
[TASK] Compact a loop expression in BackendConfigurationManager

Switch foreach() to single line while().
[TASK] Yoda CGL
[TASK] Implement own method building default view configuration for ext key

Implement an own method building the default view configuration of the
given extension key, so it can be better extended by sub-services like
fluidpages or fluidcontent, which can provide by default a deeper folder
structure. (e.g "Templates/Page" for fluidpages or "Templates/Content"
for fluidcontent).
[TASK] Make method building the default viewconfiguration protected