[BUGFIX] ExtensionManagementUtility tries to include non-existing files

Methods createExtLocalconfCacheEntry() and createExtTablesCacheEntry()
attempt to include files without checking if they exist. This causes
PHP warnings about non-existing files and looses time in a call to
GeneralUtility::getUrl().

Change-Id: Icbee0c8e6e9e0ada1dc48ac471006b6dc59adc6d
Resolves: #46074
Releases: 6.0, 6.1
Reviewed-on: https://review.typo3.org/18757
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn