diagnostics: sort package diagnostics by position

Previously, the error messages could be shown out of order.
With this patch, the errors are sorted before being shown to the user.
This makes it easier to read the error messages, and matches what the
`go` toolchain does.

testdata/errors/types.go

@@ -7,6 +7,6 @@ func main() {
 }
 
 // ERROR: # command-line-arguments
+// ERROR: types.go:4:6: a declared and not used
 // ERROR: types.go:5:6: cannot use "foobar" (untyped string constant) as int value in assignment
 // ERROR: types.go:6:2: undefined: nonexisting
-// ERROR: types.go:4:6: a declared and not used