opam var and opam option fixes when no switch is installed

[rjbou]

• Don't error when displaying if switch is not set fix opam var XXX fails when no switch is selected #5025
• Try to set a variable with option --switch <sw> fails instead of writing a wrong switch-config file
• When a field is defined in switch and global scope, try to determine the scope also by checking switch selection

the second point need to be backported to 2.1.3

[kit-ty-kate]

Looks good!

As a side note, it might be a good idea to remove OpamStateConfig.get_switch in the future. It looks like its design brings issues such as this fairly easily (If i understand what the issue was correctly)

[rjbou]

Not really, get_switch must be used carefully, but it helps to homogeneise the error & its message. The get_switch_opt function is present, and can be used. But i agree, we need to take a tour and check its use in opam, and replace needed ones.

[AltGr]

Good to merge.
Remark: the ~no_switch flag is defined but never used.

[rjbou]

Certainly a kept old implementation, it is not needed.
Just to keep code somewhere in case one day it is needed:

let with_switch ~display gt lock_kind ?no_switch st k =
  match st with
  | Some st -> k st.switch st.switch_config
  | None ->
    match OpamStateConfig.get_switch_opt () with
    | None ->
      (match no_switch with
       | None ->
         OpamConsole.error_and_exit `Configuration_error "No switch selected"
       | Some no_switch ->
         no_switch (OpamSwitch.of_string "no-switch")
           OpamFile.Switch_config.empty)