fix invalid use of subprojects

The keycodemapdb Meson subproject provides a program and a source input. Since it is a subproject, Meson wants to sandbox that and requires it to be explicitly exported. But this never happened -- instead, we manually poked at files using the actual string path "subprojects/......"

This was always a Meson sandbox violation, but Meson 0.63.0 started noticing it and erroring out.

Instead, do the right thing. Update the subproject to a version that has a meson.build with actually meaningful contents -- namely, a files variable and a found program. Then use these in order to run the needed custom_target.

In the process, it is also necessary to correct the argument ordering when running keymap-gen.