improve NuGet discovery by expanding *.proj and *.sln files

[brettfo]

This also removes a recursive search that wasn't correct; the only entry points allowed from any given directory are the following:

• *.sln // n.b., this will be expanded to regular project files
• *.proj // n.b., this will be expanded to regular project files
• *.(cs|vb|fs)proj // regular project files

This fixes some internal issues where a .sln file might be in a different directory than the projects, e.g., dependabot is started in the /solutions directory and the following files exist: /solutions/the-solution.sln and /src/project.csproj. Previously, dependabot wouldn't have been able to navigate from the /solutions directory over to /src but now as long as the project listing in the .sln is correct, then we'll pull in the correct projects.

The same applies to *.proj files as well as *.(cs|vb|fs)proj files that might have had a <ProjectReference> element to another directory.

[samtrion]

@brettfo It would certainly make sense to also consider the open feature requests during this refactoring.

#9711
#9745

[brettfo]

@samtrion There's more than just this PR to enable anything that's not .csproj/.vbproj/.fsproj so I'll leave that for the future just to keep this PR focused.