Don't patch twice the same file

[rjbou]

Patch was launched twice on the same file, and the same piece of code lead to an error with dev profile errors enabled :

File "src/client/opamAction.ml", line 305, characters 6-34:
305 |       OpamFilename.patch patch dir)
            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Error: This expression has type exn option job
       but an expression was expected of type unit

• Add a test for patching and substitution moved to Some tests #4546