[BUGFIX] Extbase BackendConfigurationManager fakes sys_template

The TypoScript parser has to load TypoScript from globals
($GLOBALS['TYPO3_CONF_VARS']['FE']['defaultTypoScript_')
and TypoScript from extension static files
(ext_typoscript_*.typoscript) at some point.

It usually adds this before the first "root" sys_template
row. This is fine in frontend.

Extbase backend modules rely on frontend TypoScript for
configuration as well. This decision from the dark ages
hurts us plenty, but is very hard to get rid of.

The extbase BackendConfigurationManager does a lot of
guesswork to calculate such frontend TypoScript: There
are various special cases, especially when an extbase
backend module is not loaded within page context.

The ext:form extension triggers such a special case:
When there is only a single page marked as "is_siteroot"
without a sys_template record, configuration from
globals is not included. The configuration is then
incomplete and the module fails.

The old TypoScript parser had a dedicated...