(This used to be T21485 on secure-dot, but it never got anywhere)

This is the basis of the client-side support for Packages:

• Installed packages are under ~/.cache/arcanist-packages/<identifier>-<version>.
• A package has a manifest file called .arcpackage.
• A package can be Requested from any config source, as packages, an array of objects.

This change includes:

• loading requested packages that are installed
• list all installed packages
• a flow to install a package (but not to configure loading it).

To configure a package, this goes in any config file:

{
  "packages": [
    {
      "identifier": "pk1",
      "version": "1.12"
    },
    {
      "identifier": "aviv.demo1",
      "version":"1.0"
    }
  ]
}

and a manifest looks like this:

{
    "identifier": "aviv.demo1",
    "version": "1.0",
    "Description": "An important demo package"
}

The --install-from-tgz flow is obviously just temporary, to be replaced later with better setup.

Also used --install-from-tgz on a package.