wrong paths on windows

We should use native path separator only in `path` attribute.
In all other places, we should use unix one, `/`.
Currently, we end up with stuff like this:

```
"./lib\\main": {
"type": "local",
"path": "C:\\Users\\Alex\\Desktop\\mozilla\\addon-sdk\\examples\\reddit-panel\\lib\\main.js",
"id": "./lib\\main",
"requirements": {
"self": "..\\..\\packages\\api-utils\\lib\\self",
"panel": "..\\..\\packages\\addon-kit\\lib\\panel",
"tabs": "..\\..\\packages\\addon-kit\\lib\\tabs",
"widget": "..\\..\\packages\\addon-kit\\lib\\widget"
}
},
"..\\..\\packages\\api-utils\\lib\\self": {
"type": "deprecated",
"path": "..\\..\\packages\\api-utils\\lib\\self.js",
"id": "..\\..\\packages\\api-utils\\lib\\self",
"warning": {
"type": "core"
},
"requirements": {
"chrome": "chrome",
"@loader/options": "@loader/options"
}
},
```