Autogenerate the extension unpackaged script

[robe]

Right now before release, we generate the unpackaged scripts used to

CREATE EXTENSION postgis FROM unpackaged;

by installing postgis with CREATE EXTENSION and then running a query to see what is in the package. strk is working on script to generate the unpackaged script by sourcing the .sql files we generate. This will simplify our packaging logic and also ensure the unpackaged is always in synch at all times.

For testing the quality of it I propose as part of installcheck

1. Count number of objects before PostGIS install
2. Install PostGIS using non-extension scripts
3. Run the generated CREATE EXTENSION postgis FROM unpackaged;
4. Run DROP EXTENSION postgis;
5. The number of objects in data install should be the same as 1