devpkg: allow multiple/non-default package outputs (#1630)

Update `devpkg.Package` to use `FlakeInstallable` instead of a URL so
that it can correctly handle the flake outputs syntax. For example,
`flake:nixpkgs#prometheus^out,cli`.

Untangling parts of `devpkg` has been tricky, so this commit focuses on
changing only the pieces necessary to replace `Package.URL` with
`Package.installable`. There's still a lot more cleanup that can be
done.

Tested by adding/removing various flakes with and without outputs
specified:

```
$ devbox add prometheus
$ devbox run -- promtool --version
promtool: command not found
Error: error running script "promtool" in Devbox: exit status 127

$ devbox add flake:nixpkgs#prometheus
$ devbox run -- promtool --version
promtool: command not found
Error: error running script "promtool" in Devbox: exit status 127

$ devbox add flake:nixpkgs#prometheus^out,cli
$ devbox run -- promtool --version
promtool, version 2.47.2 (branch: unknown, revision: unknown)
  build user:       nix@nixpkgs
  build date:       unknown
  go version:       go1.21.3
  platform:         darwin/arm64
  tags:             builtinassets

$ devbox add path:$HOME/src/tailscale/tailscale
```

Related to #1431.

Author: gcurtis