* Trace all external pointers going through a first map

Currently, MapVisitor only detects maps with external pointers as
values if the value was directly passed from a function's argument.
For example, in the following, the rewriter is currently unable to
detect currsock has an external pointer as value because an
intermediate variable is used instead of passing directly sk as the
map's value.

    int test(struct pt_regs *ctx, struct sock *sk) {
        u32 pid = bpf_get_current_pid_tgid();
        struct sock **skp = &sk;
        currsock.update(&pid, skp);
        return 0;
    };

With this commit, MapVisitor is able to trace any external pointer
derived from the function's argument and used as a map value. This
commit breaks the ProbeVisitor traversal in two distinct traversals.
The first rewrites dereferences of external pointers originating
from function's arguments and helpers, while the second rewrites only
dereferences of external pointers passed through maps.
Maps with external pointers as values are identified between the two
ProbeVisitor traversals.

* New tests for external pointers passed through maps

test_ext_ptr_maps_reverse ensures dereferences are correctly replaced
even if the update happens after the lookup (in the order of
MapVisitor traversal).
test_ext_ptr_maps_indirect ensures the rewriter is able to trace
external pointers used as map values even if using an intermediate
variable.