Core support for "extensions", which are packages of SQL objects.

This patch adds the server infrastructure to support extensions.
There is still one significant loose end, namely how to make it play nice
with pg_upgrade, so I am not yet committing the changes that would make
all the contrib modules depend on this feature.

In passing, fix a disturbingly large amount of breakage in
AlterObjectNamespace() and callers.

Dimitri Fontaine, reviewed by Anssi Kääriäinen,
Itagaki Takahiro, Tom Lane, and numerous others

src/backend/catalog/pg_namespace.c

@@ -38,6 +38,7 @@ NamespaceCreate(const char *nspName, Oid ownerId)
 	Datum		values[Natts_pg_namespace];
 	NameData	nname;
 	TupleDesc	tupDesc;
+	ObjectAddress myself;
 	int			i;
 
 	/* sanity checks */
@@ -73,9 +74,17 @@ NamespaceCreate(const char *nspName, Oid ownerId)
 
 	heap_close(nspdesc, RowExclusiveLock);
 
-	/* Record dependency on owner */
+	/* Record dependencies */
+	myself.classId = NamespaceRelationId;
+	myself.objectId = nspoid;
+	myself.objectSubId = 0;
+
+	/* dependency on owner */
 	recordDependencyOnOwner(NamespaceRelationId, nspoid, ownerId);
 
+	/* dependency on extension */
+	recordDependencyOnCurrentExtension(&myself);
+
 	/* Post creation hook for new schema */
 	InvokeObjectAccessHook(OAT_POST_CREATE, NamespaceRelationId, nspoid, 0);