Feat: Allow multiple sources in plugins

[axtloss]

Adds support for defining more than one source in the meson and make plugins as well as some improvements in systemd-sysext

[taukakao]

Having a different syntax for the sources field in just this module seems very inconsistent thus counter intuitive.

Would an "additionalSources" field or something work here? That would also not require a breaking change in the vib syntax.

[axtloss]

Having a different syntax for the sources field in just this module seems very inconsistent thus counter intuitive.

what do you mean? that's already a thing in the shell module and this pr adds it to the rest

[taukakao]

that's already a thing in the shell module and this pr adds it to the rest

I see. But should it then not also be added for the go and apt plugins for example?

[axtloss]

apt possibly yes.
not go though, all it does is compile a binary using go and install it, any extra files couldn't even be used by the plugin, with the meson, make and shell command they can

[taukakao]

Fair enough. I think it should be as consistent as possible, of course excluding plugins which it doesn't make sense for.