move process implementations to separate modules

[bobvanderlinden]

As discussed on Discord this is a proposal implementation for splitting up the process-managers into separate modules.

I must say that this became a bit bigger than I initially anticipated, but it seemed like the refactorings were still useful and backwards-compatible.

There are a few todo's in here to propose further changes, but first wanted to get feedback on this proposal as-is.

While working on this I also felt it might be a good idea to move generating the Procfile and env-file into a pkgs.formats.procfile and pkgs.formats.envfile. That way these implementations can be reused, but do not need to reside as options (process-compose doesn't use them for instance, as do a number of other non-procfile-based process managers). Not really needed to worry about right now though.

[jlesquembre]

@bobvanderlinden We need to keep the port and tui options in the CLI for process-compose:

devenv/src/modules/processes.nix

Lines 56 to 60 in eae0025

	process-compose = ''
	${pkgs.process-compose}/bin/process-compose --config ${config.procfile} \
	--port ''${PC_HTTP_PORT:-${toString config.process.process-compose.port}} \
	--tui=''${PC_TUI_ENABLED:-${toString config.process.process-compose.tui}} up "$@" &
	'';

I created a PR:
#700