Don't include scope for string-valued bin

[charlespierce]

We support both a hash and a string for the bin entry in package.json. When using a string, it's interpreted as the path to the binary, while the name for the binary is the name of the package. However, to match the behavior of the package managers, when the package is scoped, we should not include the scope in the binary name, only the unscoped package name.