Remove package scope from default binary name

[charlespierce]

Closes #963

Info

• When the bin field in package.json is a string (as opposed to a hash), that's interpreted as the path to the binary and the package name is used as the name of the binary.
• However, when the package is scoped (e.g. @scope/package-name), then the binary should be named after the package without the scope (package-name in this example).
• Including the scope causes problems when creating shims, as the / is interpreted as a filesystem directory separator.

Changes

• Updated the logic that handles string-valued bin to strip off any scope when setting the name of the binary.

Tested

• Added unit tests to confirm that removing the scope works as expected
• Manual testing to confirm that a scoped package with a string bin can be installed correctly, with the appropriate binary name.

[dherman]

Very clear and readable! 🚢