Extensions: address some follow-ups related to using directives

[jcouv]

Addresses parts of #78827

Note: an implementation method for a non-static new extension method is an extension method, and therefore not imported as a static method (existing behavior, see IsValidLookupCandidateInUsings) and cannot be invoked directly. It can only be called by extension invocation.

Relates to test plan #76130

[jjonescz]

Shouldn't we check !IsReferenceType || !IsStatic || IsGenericType inside MightContainExtensionMethods instead? #WontFix

[jcouv]

Yes, that would make sense.
As part of this PR, I'd identified a couple other questions/follow-ups regarding MightContainExtensionMethods. I captured them to this follow-up issue and captured your suggestion there as well.
I prefer to keep this PR very focused and keep this moved code as-is for now. I'll take a stab at this in next PR, as it's bothering me too.

[jcouv]

Looked around a bit and it's already clear the answer is not obvious, so would rather not open a possible can of worms. See PENamedTypeSymbol.MightContainExtensionMethods:

        public override bool MightContainExtensionMethods
        {
            get
            {
                var uncommon = GetUncommonProperties();
                if (uncommon == s_noUncommonProperties)
                {
                    return false;
                }

                if (!uncommon.lazyContainsExtensionMethods.HasValue())
                {
                    var contains = ThreeState.False;
                    // Dev11 supports extension methods defined on non-static
                    // classes, structs, delegates, and generic types.
                    switch (this.TypeKind)
                    {
                        case TypeKind.Class:
                        case TypeKind.Struct:
                        case TypeKind.Delegate:
                        ...

[AlekseyTs]

LGTM (commit 1)