move scala-cli from the 'contrib' to the 'default' coursier install channel

[mtk]

i was surprised to find out that scala-cli was not part of the default install channel for coursier. it seemed to violate the sacred principle of least astonishment :-). i've opened a coursier issue (a suggestion from chris kipp), requesting discussion about moving it from 'contrib' to 'default'. i mentioned there that i would open a discussion item here to make sure that krzysztof & others knew about the possible change. i can't see any cons, only pros, but i'm an outsider wandering in the wilderness :-).

[mtk]

here is the coursier issue: coursier/coursier#2392