Implement proper UFCS name look up

[9rnsr]

In basic, UFCS name search looks for only symbols in module scope.
If the enclosing scopes have local imports, the imported overload-set and selected/renamed symbols should also be considered in searching.

In my opinion, UFCS should not see the locally declared symbols (in statement scope and aggregate member scope). So, currently, this is a final step for the UFCS implementation.

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Fixed bugzilla issue:
Issue 6185 - Include non-global functions when resolving UFCS

[ghost]

What about static methods, will they be taken into account? E.g.:

struct S
{
    static int square(int val) { return val * val; }
    int s() { return 2.square; }  // OK or NG?
}

[9rnsr]

As I wrote in the summary, I finally decided that UFCS should only see the symbols declared in module scope, and I implemented it.
All of static methods in aggregate scope are not declared in the module scope, so they are not considered.

It looks like unneeded limitation, but I think this is reasonable. My reasons are:

1. UFCS is not for syntactic sugar.
   The static method is properly accessible from the method of aggregates. You can call it with normal function call syntax. UFCS is not necessary there.
2. It is not the purpose of UFCS.
   As far as I know, the original purpose of UFCS is for the ability to add retrofit auxiliary methods/properties to the existing types. Such auxiliary utilities should be declared in module scope, to make them accessible from other modules.
3. Keep compiler implementation simple.
   static methods and non-static ones are overloadable in D. Until resolve overloading, we cannot detect the function is really static or not. In name lookup phase, the elements of overloaded symbols should not be considered.