make coqdep canonicalize paths from the command line

- logical paths given to -R and -I should be split
  on periods.
- it also seems like giving an empty string should
  result in the empty path rather than the singleton
  path with an empty string as an identifier.
Update infer_conv to record trivial Prop <= Type i constraints that are needed during
unification.
- Fix in kernel conversion not folding the universe constraints
correctly when comparing stacks.
- Disallow Type i <= Prop/Set constraints, that would otherwise allow
constraints that make a universe lower than Prop.
- Fix stm/lemmas that was pushing constraints to the global context,
it is done depending on the constant/variable polymorphic status now.
- Adapt generalized rewriting in Type code to these fixes.