Introducing default cli

[rjbou]

Some deprecated options need to still be accepted (cf. #4503). The default cli is here to allow them when used with no specified cli (not from environment or from option --cli). Opam prints then a removal warning notice.
The default version is set to 2.0, it will be upgraded at each major release.
Default cli options are ftm opam config deprecated commands.

Example with config var:

$ opam config var prefix
[WARNING] var is deprecated since version 2.1 of the opam CLI, but version 2.1 has been requested. Use opam var instead or set OPAMCLI environment variable to 2.0.
.opam/switch

$ opam config var prefix --cli 2.0
.opam/switch

$ opam config var prefix --cli 2.1
opam: var was removed in version 2.1 of the opam CLI, but version 2.1 has been requested. Use opam var instead or set OPAMCLI environment variable to 2.0.

$ OPAMCLI="2.0" opam config var prefix
.opam/switch

$ OPAMCLI="2.1" opam config var prefix
opam: var was removed in version 2.1 of the opam CLI, but version 2.1 has been requested. Use opam var instead or set OPAMCLI environment variable to 2.0.

[dra27]

Lovely, thanks! The only changes are the misspelling of previously_str and the deprecation warning can be re-worded. Semantically it's doing exactly what's needed, and I very much like OpamCLIVersion.default. Perhaps it could be automatically the major version of opam?