fossology_license: Improve add query endpoint

This also clarifies the intent.

sql/upgrades/130.sql

+-- SWH Indexer DB schema upgrade
+-- from_version: 129
+-- to_version: 130
+-- description:
+
+insert into dbversion(version, release, description)
+values(130, now(), 'Work In Progress');
+
+create or replace function swh_content_fossology_license_add(conflict_update boolean)
+    returns void
+    language plpgsql
+as $$
+begin
+    -- insert unknown licenses first
+    insert into fossology_license (name)
+    select distinct license from tmp_content_fossology_license tmp
+    where not exists (select 1 from fossology_license where name=tmp.license)
+    on conflict(name) do nothing;
+
+    if conflict_update then
+        insert into content_fossology_license (id, license_id, indexer_configuration_id)
+        select tcl.id,
+              (select id from fossology_license where name = tcl.license) as license,
+              indexer_configuration_id
+        from tmp_content_fossology_license tcl
+            on conflict(id, license_id, indexer_configuration_id)
+            do update set license_id = excluded.license_id;
+        return;
+    end if;
+
+    insert into content_fossology_license (id, license_id, indexer_configuration_id)
+    select tcl.id,
+          (select id from fossology_license where name = tcl.license) as license,
+          indexer_configuration_id
+    from tmp_content_fossology_license tcl
+        on conflict(id, license_id, indexer_configuration_id)
+        do nothing;
+    return;
+end
+$$;
+
+comment on function swh_content_fossology_license_add(boolean) IS 'Add new content licenses';

swh/indexer/sql/30-swh-schema.sql

@@ -14,7 +14,7 @@ create table dbversion
 );
 
 insert into dbversion(version, release, description)
-      values(129, now(), 'Work In Progress');
+      values(130, now(), 'Work In Progress');
 -- Computing metadata on sha1's contents
 
 -- a SHA1 checksum (not necessarily originating from Git)

swh/indexer/sql/40-swh-func.sql

@@ -228,13 +228,14 @@ begin
     on conflict(name) do nothing;
 
     if conflict_update then
-        -- delete from content_fossology_license c
-        --   using tmp_content_fossology_license tmp, indexer_configuration i
-        --   where c.id = tmp.id and i.id=tmp.indexer_configuration_id
-        delete from content_fossology_license
-        where id in (select tmp.id
-                     from tmp_content_fossology_license tmp
-                     inner join indexer_configuration i on i.id=tmp.indexer_configuration_id);
+        insert into content_fossology_license (id, license_id, indexer_configuration_id)
+        select tcl.id,
+              (select id from fossology_license where name = tcl.license) as license,
+              indexer_configuration_id
+        from tmp_content_fossology_license tcl
+            on conflict(id, license_id, indexer_configuration_id)
+            do update set license_id = excluded.license_id;
+        return;
     end if;
 
     insert into content_fossology_license (id, license_id, indexer_configuration_id)