Removed browser field dep breaks cached build

[calvinwiebe]

Observed in benbria/loop and in a verification test: If there is a module listed in the browser field of package.json, and the file that field points to changes, and the original file is removed, the build will break with something like:

Error: ENOENT, open '/Users/calvinwiebe/dev/uber-test/foo.js'

For instance:
package.json:

browser: {'foo': './foo.js'}

Build and save cache.
Remove ./foo.js and point browser field to a different file. Such as:

browser: {'foo': './inner/foo.js'}

There will be something in the cache like this:

"deps":{"foo":"/Users/calvinwiebe/dev/uber-test/foo.js"}

And everything will die.

Removing a normal require path and file will update the cache accordingly. The solution to this might be a bit involved, as we'd need to walk up the dependency tree.