Install packages with an explicit path with an unknown package type

[jcnventura]

At the moment, this plugin only affects packages added to installer-types. While that makes a lot of sense for type:xyz install paths, it's hard to understand why the plugin is not able to install a named package to an explicit path. As per the example given in the README.md

{
    "extra": {
        "installer-paths": {
            "special/package/": ["my/package"],
        }
    }
}

The above should be able to install my/package to special/package with whatever package type that package has declared. This would probably simplify a lot of confusion on how to use the plugin such as the one in #14.