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
```