Dune can't use local OPAM switch in context declaration

[argent-smith]

Expected Behavior

Dune uses the local OPAM switch same as global switch.

Actual Behavior

Dune gives the error message

Reproduction

1. create a local opam switch in your project directory:

cd boinc-controller && opam create . 4.10.0 -y

2. write this dune-workspace file:

(lang dune 2.4)
(context
 (opam
  (switch /Users/paul/Development/own/mirage/boinc-controller)
  (merlin)))

3. run dune build

4. see the error message:

⟩ dune build
File "dune-workspace", line 4, characters 10-61:
4 |   (switch /Users/paul/Development/own/mirage/boinc-controller)
              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Error: "/Users/paul/Development/own/mirage/boinc-controller" is an invalid
context name.

Specifications

• Version of dune (output of dune --version): 2.4.0
• Version of ocaml (output of ocamlc --version): 4.10.0
• Operating system (distribution and version): Mac OS 10.14.6 Mojave

[ghost]

Fix in #3265