Contract level checker: Disallow free function redefinition and alias

shadowing another free function

Co-authored-by: chriseth <chris@ethereum.org>

test/libsolidity/syntaxTests/multiSource/reimport_imported_function.sol

@@ -0,0 +1,11 @@
+==== Source: s1.sol ====
+function f() pure returns (uint) { return 1337; }
+==== Source: s2.sol ====
+import {f as g} from "s1.sol";
+==== Source: s3.sol ====
+import {g as h} from "s2.sol";
+contract C {
+  function foo() public pure returns (uint) {
+    return h();
+  }
+}