Allow opam source to run without a switch, add a `--no-switch' option

[zapashcanon]

@rjbou Update:

• Added a --no-switch option fix support opam source --no-switch #4858
• Run on a non switch mode if there is the option or if switch list is empty

---

Here's a draft to fix #4809 as asked by @dra27.

It doesn't typecheck:

$ make
dune build --profile=release --root .  --promote-install-files -- opam-installer.install opam.install
File "src/client/opamCommands.ml", line 3358, characters 44-45:
3358 |           OpamAction.prepare_package_source t nv dir @@| function
                                                   ^
Error: This expression has type
         OpamStateTypes.unlocked OpamStateTypes.switch_state
       but an expression was expected of type
         OpamStateTypes.rw OpamStateTypes.switch_state
       Type
         OpamStateTypes.unlocked = [ `Lock_none | `Lock_read | `Lock_write ]
       is not compatible with type OpamStateTypes.rw = [ `Lock_write ]
       The second variant type does not allow tag(s) `Lock_none, `Lock_read

But currently, there's no simple way to get a lock for the virtual state so I don't know how to fix this...

cc @AltGr @rjbou

[kit-ty-kate]

Just some cosmetic suggestions but otherwise LGTM (the github "hide whitespace" option was helpful ^^)

[zapashcanon]

If you (@rjbou or @kit-ty-kate) think it's ready for merge, then I'm fine with it.

[kit-ty-kate]

@zapashcanon actually, if you have some spare time, could you add some tests to a new tests/reftests/source.test file? I just realized this PR can be tested easily and that opam source is not tested at all

[rjbou]

test added