At the moment, npm will NOT install a dependency that is both a
production and a development dependency in the same tree if
`--only=dev`.

Consider the following scenario:

- `package1` has `prod-dependency` in `dependencies`
- `package1` has `dev-dependency` in `devDependencies`
- `prod-dependency` has `sub-dependency` in `dependencies`
- `dev-dependency` has `sub-dependency` in `dependencies`

So both `prod-dependency` and `dev-dependency` directly depend on
`sub-dependency`. Since `sub-dependency` is required by at least one
production dependency, then npm won't consider it as "only a dev
dependency", and therefore running `npm install --only=dev` will result
in the following tree:

```
package1/
    node_modules/
        dev-dependency
```

Notice that `sub-dependency` has ben completely omitted from the tree,
even though `dev-dependency` clearly requires it, and therefore it will
not work.

This commit makes `--only=dev` always install required dependencies of
dev dependencies, so you never end up with a broken tree.

For a more real-world reproducible example, try to run `npm install
--only=dev` in
https://github.com/resin-io/etcher/tree/a338c6e60a10aad00008febfe92e2556dcf586c6.
The resulting tree will be completely broken, but we would not have
identified this if several install scripts wouldn't fail as a result.

Signed-off-by: Juan Cruz Viotti <jv@jviotti.com>