code and registrations cleanup

[BuckarooBanzay]

contents of this PR:

• proper luacheck ci
• various import-cleanups
• game-dependent materials are now exposed in pipeworks.materials
• moved a big chunk of common functions from init.lua to common.lua
• created a new directory for tube modules
• added pipeworks.register_embedded_tube(nodename, opts) to be used in other mods

(sorry for the big PR, i originally wanted to make a PR for the elbow-tube but was distracted somehow :shrug:)

just for fun (not included here)

```lua pipeworks.register_embedded_tube(":pipeworks:mese_block_embedded_tube", { description = "Airtight mese embedded tube", base_texture = "default_mese_block.png", base_ingredient = "default:mese" }) ``` 

[BuckarooBanzay]

i really would not like materials to be exposed. this should properly be done with a mod/library like sounds, since this code is already semi duplicated across our mods with homedecor, steel, here, at least

materials is now available in pipeworks only (via loadfile) :+1: