[BUGFIX] Load ext_emconf information in extension installation

With patch https://review.typo3.org/50427/ TER information were not
loaded for extension information. Unfortunately this also skips loading
the dependencies which results in uninstalled dependencies. This patch
introduces a new function to only load needed (and available)
information from ext_emconf file for processing a correct extension
installation/activation.

Resolves: #78540
Releases: master, 7.6, 6.2
Change-Id: I16bddc91cb2523488cedc2293936eeb8aa2eb577
Reviewed-on: https://review.typo3.org/50484


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Michael Oehlhof <typo3@oehlhof.de>
Tested-by: Michael Oehlhof <typo3@oehlhof.de>
Reviewed-by: Andreas Fernandez <typo3@scripting-base.de>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Reviewed-by: Markus Klein <markus.klein@typo3.org>
Tested-by: Markus Klein <markus.klein@typo3.org>