Fix templates not being picked up by `can.view()` when using `steal('som...

When using the SystemJS ext plugins, `can.view(url)` isn't finding the templates.

In `stealconfig.js`, I have:

    ...
    ext: {
        css: "$css",
        less: "$less",
        ejs: "can/view/ejs/system",
        mustache: "can/view/mustache/system",
        stache: "can/view/stache/system"
    },
    ...

I load the template with steal:

    steal('example/template.mustache!', ...)

I can see it has been added to `can.view.cached["example/template.mustache"]`.

But then when I call `can.view('example/template.mustache')`, it doesn't see the cached template and tries to fetch it (which fails for another reason). 

It looks like `can/view/view.js` doesn't lookup the cache with `url`, instead uses `id` where `id = toId(url)`. 

I haven't checked stache templates.