Allow to parse opam 2.1 switch import files containing extra-files

opam 2.1 used the Base64 module from extlib which does not seem to pad encoded outputs.
In contrast, opam 2.2 uses the Base64 module from the base64 library
(after extlib's Base64 module was removed in extlib.1.7.8) which by default pads outputs
and requires padded inputs when decoding them

master_changes.md

@@ -28,6 +28,7 @@ users)
 ## Remove
 
 ## Switch
+  * Allow to parse opam 2.1 switch import files containing extra-files [#5943 @kit-ty-kate - fix #5941]
 
 ## Config
 

src/client/opamSwitchCommand.ml

@@ -398,7 +398,7 @@ let import_t ?ask importfile t =
     OpamPath.Switch.extra_files_dir t.switch_global.root t.switch
   in
   OpamHash.Map.iter (fun hash content ->
-      let value = Base64.decode_exn content in
+      let value = Base64.decode_exn ~pad:false content in
       let my = OpamHash.compute_from_string ~kind:(OpamHash.kind hash) value in
       if OpamHash.contents my = OpamHash.contents hash then
         let dst =