Throw RuntimeException when ambiguous reference was found.

Let's say that I have project with two classes with the same name, but in different directories, e. g.:

`src/A.php`
```
<? class A { }
```
and `src/otherDir/A.php`
```
<? class A { }
```
and `composer.json`
```
{
    "autoload": {
        "classmap": ["src/"]
    }
}
```
I do `composer install` and I would expect that composer warns me, that the project contains ambiguous reference to `A` class.