Support dots in resolver names for namespaces

[odelalleau]

Is your feature request related to a problem? Please describe.

Introducing new resolvers in libraries may clash with user-defined custom resolvers.

Describe the solution you'd like

Supporting dots in resolver names to denote namespaces would avoid such clashes, e.g., env could become oc.env (although this specific change to the env resolver is not part of this issue).

Nested namespaces will also be supported (ex: ${ns1.ns2.func:123}).

Describe alternatives you've considered

Also considered _.

Additional context

Related to discussion in #383 (comment)