flag: include source of original definition in redefinition error

<pre>If a flag is redefined, it may be due to a transitively imported package you're not
aware of holding the original definition. To help diagnose this, it might be worth
remembering where a flag was defined, although doing so will  require the flag package
to import runtime and other such complexities. I'm not sure it's worthwhile, but I
wanted to record the idea here.</pre>
