Improved package support/modularization

The way the layer currently handles packages is inconsistent. Packages to be installed via apt are placed into layer.yaml, but python packages are in wheelhouse.txt, and there's no support for other packaging formats, such as snaps or npm.

A more modular approach would allow developers to write support for a particular packaging format, with a consistent way to install them. Two possible formats of layer.yaml to support this idea:

``` yaml
options:
basic:
packages:
- apt:traceroute
- pip:paramiko==2.0.1
- npm:grunt
- npm:grunt-cli
- snap:hello-world
```

``` yaml
options:
basic:
packages:
apt:
- traceroute
pip:
- paramiko==2.0.1
npm:
- grunt
- grunt-cli
snap:
- hello-world
```

Comienza leyendo cómo se gestionan actualmente las entradas de paquetes en layer.yaml y cómo se leen los paquetes de Python desde wheelhouse.txt. Compara los formatos propuestos de layer.yaml con el comportamiento existente y determina las interfaces de packaging compatibles; se considerará completado cuando exista un enfoque modular acordado para los formatos de paquete indicados.

Escrito por el modelo de indexación a partir del texto del issue.