[TAKS] Make getExtensionFromT3xFile public

In order to make the ext:coreapi import command workable make the
method UploadExtensionFileController::getExtensionFromT3xFile()
accessible.

The command looks like that:

./typo3cms extensionapi:import dummy.t3x

Resolves: #59999
Releases: 6.2, 6.3
Change-Id: I21e734d1ecae2e7c8e7c3f72e129ef1f57532886