kvdb/sqlbase: use positive+negative build tags for new sql error parsing

In this commit, we use exhaustive build tags to ensure that we can
always build the `sqlbase` package, independent of the set build tags.
To do this, we move the type declarations _into_ the parsing functions.
This then allows us to create two versions for each db: with the db, and
without it.

To avoid a module tag round trip to get this working, we use a local
replace for now. Once this is merged in, we can do the tag (along side
rc3), then remove the replace.

kvdb/sqlbase/sqlerrors_no_postgres.go

@@ -0,0 +1,9 @@
+//go:build !kvdb_postgres
+
+package sqlbase
+
+// parsePostgresError attempts to parse a postgres error as a database agnostic
+// SQL error.
+func parsePostgresError(err error) error {
+	return nil
+}