Ability to write out package manifests as source code, providing more flexibility for things like mechanical package manifest generation.

Note that this is _not_ suitable for editing the package manifest, since that would need to preserve whitespace and comments.  Rather, this is for things like automatically generating packages from test sets, or for generating the initial configuration of packages (e.g. a more flexible `package init`).

This is still a WIP, and still needs support for:
- resources
- build settings
- binary targets

Additionally, there are FIXMEs to resolve and many more unit tests to add.

rdar://70271810

Author: abertelrud