default entry point for extension + sqlite db update

Wikimedia CI system expect extension to have a default entry point named
a like the extension. A symlink is good enough for it.

The ExtensionSchemaUpdate hook was only acting for MySQL backend, added
in the sqlite database backend as well.

Change-Id: I53ec2c3d78c4110f5f6530c7c52120e70a01e667
Merge "default entry point for extension + sqlite db update"