Normalize actions signature (take 2)

[blaggacao]

From:

{
  currentSystem,
  fragment, # <system>.<cell>.<block>.<target>
  fragmentRelPath, # <cell>/<block>/<target>
  target,
}: []

To:

{
  root, # <cellsFrom>
  target, # any
  cursor, # ["<system>" "<cell>" "<block>" "<target>"]
  currentSystem,
}: []

Rationale:

• root + cursor[1:] allows to reason and/or interact with the mutable repository layout
• currentSystem abstracts that the action (not the target) must act on the host system This is not the same as cross-compilation!
• cursor allows to construct a unique path inside the PRJ_* folders
• cursor also allows to reason about the flake fragment