This replaces all deprecated function usage by alternative ones based on
a version switch enacted at compile time, preventing all warnings.

This will likely introduce some possible runtime errors in using a
Rebar3 compiled on OTP-20 or OTP-21 back in versions 19 and earlier, but
we can't really work around that.

A bunch of dependencies have been updated to support OTP-21 without
warnings as well.

Similarly to PATH env variable, this allows to pass paths to bare
compiler which do not fit nicely into a single wildcard structure.
Colon (":") is used as separator.

This provides more flexibility when rebar is run in offline/hermetic
environment, e.g. #958 and #1281.

- Crashes in providers lib when no providers in a namespace are bare
- Making sure bareness matches semantics; i.e. a bare provider is
  visible, a non-bare provider is hidden.