`dotnet run` should have a better warning when a user passes in a loose file and we're operating in project-mode

[baronfel]

Describe the bug

dotnet/website-feedback#395

When a user passes in a file as the 'project' to build and this isn't a single-file-runnable file, we operate in 'project' mode. In that mode we try searching for a project in the directory specified by ./Program.cs in this linked issue. We should have a more clear error here: if a user passes in something file-like and there's a) a matching file in CWD and b) we're in 'project' mode, we should tell them to not specify the project (or better yet, just run the project!).

To Reproduce

See linked issue.

Exceptions (if any)

Further technical details

• Include the output of dotnet --info
• The IDE (VS / VS Code/ VS4Mac) you're running on, and its version

[claudiaregio]

Thanks @baronfel :)