Add C# *.packages.config support

[cd-work]

This adds support for packages.config files for the C# ecosystem under the new NugetConfig lockfile name.

The file format itself seems to be relatively simple, with just a list of package names and optional versions. The current test fixture is based on examples of this lockfile found on https://github.com.

Both packages.config on its own and *.packages.config are supported as valid names. Since this seems to be a lockfile there is no need for lockfile generation.

[maxrake]

Can the files be named as *packages.config as well? Or, will there always be a . first (*.packages.config)? I plan to look at this PR in more detail next week, but keeping someone else on the reviewer list who knows C# better is recommended.

[cd-work]

Can the files be named as packages.config as well? Or, will there always be a . first (.packages.config)?

https://github.com/search?q=path%3A**%2F*packages.config+language%3AXML&type=code suggests to me that only packages.config and *.packages.config are valid. This makes sense to me because it matches the new lockfile version and it's just generally sensible.