build: change the generation of manifest.json
build: change the generation of manifest.json

Instead of our substitution rule being used to transform

  pkg/*/manifest.json.in

into

  pkg/*/manifest.json

and then a separate copy rule being used to copy the file into

  dist/*/manifest.json

just do the substitution directly.

This solves two minor issues:

  - the pkg/*/manifest.json files were being treated by make as
    non-precious intermediates, which means that you'd often see them
    being deleted as part of a large `rm` at the end of a fresh build

  - for some reason, webpack-make is writing the manifest.json file for
    networkmanager (and only networkmanager) to its Makefile.deps,
    resulting in make *not* deleting the file as a non-precious
    intermediate.  This had the effect that we'd see spurious copies of
    the file sometimes.

martinpitt/cockpit,martinpitt/cockpit,martinpitt/cockpit,mvollmer/cockpit,martinpitt/cockpit,cockpit-project/cockpit,mvollmer/cockpit,mvollmer/cockpit,garrett/cockpit,cockpit-project/cockpit,garrett/cockpit,mvollmer/cockpit,garrett/cockpit,cockpit-project/cockpit,martinpitt/cockpit,cockpit-project/cockpit,mvollmer/cockpit,garrett/cockpit,garrett/cockpit,cockpit-project/cockpit