make the stub a bit more well-rounded by exporting files

Provide variables for:
- the found program keymap-gen
- the CSV mapping table

and for enhanced convenience, override keymap-gen

This allows grabbing the variables from another Meson project without
futzing with submodule paths, something that Meson doesn't really
diff --git a/ b/
index eb9416b..6d263aa 100644
--- a/
+++ b/
@@ -1 +1,6 @@
+project('keycodemapdb', meson_version: '>=0.46.0')
+keymap_gen = find_program('tools/keymap-gen')
+meson.override_find_program('keymap-gen', keymap_gen)
+keymaps_csv = files('data/keymaps.csv')