rename sql directory to lib-sql

Also introduces a separate constant for the sql directory, so that
it can be put separately from the rest of the data if required.

lib-sql/postcode_tables.sql

@@ -0,0 +1,15 @@
+DROP TABLE IF EXISTS gb_postcode;
+CREATE TABLE gb_postcode (
+    id integer,
+    postcode character varying(9),
+    geometry geometry,
+    CONSTRAINT enforce_dims_geometry CHECK ((st_ndims(geometry) = 2)),
+    CONSTRAINT enforce_srid_geometry CHECK ((st_srid(geometry) = 4326))
+);
+
+DROP TABLE IF EXISTS us_postcode;
+CREATE TABLE us_postcode (
+    postcode text,
+    x double precision,
+    y double precision
+);