Added require.defined() to replicate require.js

[nicjansma]

I use the (undocumented) RequireJS require.defined() function in my web app to dynamically check if the current view has any code to run. For example, I have AMD modules that are named like views/foo, and my app.js checks to see if views/foo exists. If so, it calls .init() on that module.

For example:

if (require.defined('views/' + viewName)) {
    require('views/' + viewName).init();
}

The require.defined() function isn't exposed in the almond wrapper, just require._defined (a map of views).

This PR adds a simple require.defined() function to match requirejs's.

[ooxi]

While almond is aiming at providing a minimal AMD implementation I think the patch is both small and is useful enough to justify the addition.

[nicjansma]

Any other concerns? Very simple change :)

[jrburke]

This is not quite enough: the one in requirejs allows using relative IDs, require.defined('./something'). For that to work, it is best to do this work in makeRequire, and likely using makeMap internally to the defined call to get the absolute ID. Relevant pointer from require.js code

If you want to look at updating to that pathway, we can re-evaluate. I am still a bit on the fence though. As @ooxi mentioned, this is API provider is supposed to be small for "just bundle up some modules to use as a unit" sort of thing, where I would not expect require.defined() to be useful. Each one of these things that are added increases the download size of almond for everyone.

[jrburke]

Closing for now given previous comments.