When creating an extension, the same type of dependency is used when
registering a dependency to a schema and required extensions.  This
improves the code so as those dependencies are not recorded one-by-one,
but grouped together.  Note that this has as side effect to remove
duplicate dependency entries, even if it should not happen in practice
as extensions listed as required in a control file should be listed only
once.

Extracted from a larger patch by the same author.

Author: Daniel Dustafsson
Discussion: https://postgr.es/m/20200629065535.GA183079@paquier.xyz