Support NuGet lockfiles (Updated)

[na1307]

Fix #1303

Since #6031 was last updated, Dependabot NuGet logic has been rewritten as C#, so I've put this PR up.

I'm familiar with C#, but not with Ruby. I don't know how to add a test for this. Any advice is welcome.

[mheguy-flo]

The linting failure for fetch_files is because your changes pushed the complexity over the limit.
You should be able to replace this block:

        fetched_files = []
        fetched_files += project_files
        fetched_files += directory_build_files
        fetched_files += imported_property_files

        fetched_files += packages_config_files
        fetched_files += nuget_config_files
        fetched_files += packages_lock_files
        fetched_files << global_json if global_json
        fetched_files << dotnet_tools_json if dotnet_tools_json
        fetched_files << packages_props if packages_props

with this:

  fetched_files = [
    *project_files,
    *directory_build_files,
    *imported_property_files,
    *packages_config_files,
    *nuget_config_files,
    *packages_lock_files,
    global_json,
    dotnet_tools_json,
    packages_props
  ].compact

and that should reduce the complexity.
We use the splat operator * instead of appending to a list.
And we call compact to remove nils instead of checking individually.