Generate internal dispatch only for functions that might actually get called via pointers

- This also adds support for internal library calls as a side-effect since they'll now be pulled into the internal dispatch automatically.

test/libsolidity/semanticTests/virtualFunctions/internal_virtual_function_calls_through_dispatch.sol

@@ -0,0 +1,26 @@
+contract Base {
+    function f() internal returns (uint256 i) {
+        function() internal returns (uint256) ptr = g;
+        return ptr();
+    }
+
+    function g() internal virtual returns (uint256 i) {
+        return 1;
+    }
+}
+
+
+contract Derived is Base {
+    function g() internal override returns (uint256 i) {
+        return 2;
+    }
+
+    function h() public returns (uint256 i) {
+        return f();
+    }
+}
+
+// ====
+// compileViaYul: also
+// ----
+// h() -> 2