Issue Description:

To create a new C library package, I'd like to be able to do something like:

swift package init --language c

The directory structure could be like:

├── Package.swift
└── Sources
    └── Hello
        ├── include
        │   └── hello.h
        └── hello.c

If I also specify a type other than library, I could be shown an error like:

error: If language is C, then type must be library.

The output of swift package init --help could include a line like:

  --language   swift|c [default: swift]

Bonus: I'd also be able to specify language via Xcode: File > New > Package...