honor package category for paludis packages

the package/paludis provider ignored the package category during
`load_current_resource`. Because of this, it was not possible to install
multiple package with same name in different categories

Example:
```
  * package[app-vim/screen] action install
     app-misc/screen 4.0.3-r6
  * gentoo
    app-misc/screen 4.0.3-r6 installed
      (up to date)
```