Fix #2725 by calling load_context/1 in the unspecified branch of strip_module/3

This fixes #2725, by making it so that `strip_module(Pred, M, P), call(M:P)`
doesn't throw an `instanciation_error` when `Pred` isn't in the form `module:predicate`.

Now, `strip_module(hello, M, P)` will call `load_context(M)`, which unifies `M`
with the topmost module (or `user`).

Two new test cases are added: issue2725.pl, which tests the minimal case id(X) --> X.
and the strip_module(P, M, _), call(M:P) scenario, and module_resolution,
which tests the behavior of strip_module in a few scenarios.

src/loader.pl

@@ -695,10 +695,9 @@ strip_module(Goal, M, G) :-
     (  MQ = specified(M) ->
        true
     ;  MQ = unspecified,
-       true
+       load_context(M)
     ).
 
-
 :- non_counted_backtracking strip_subst_module/4.
 
 strip_subst_module(Goal, M1, M2, G) :-