[Idea] Shorthand project reference by convention

_From @faniereynders on July 26, 2018 8:49_

Given the following `ItemGroup` in a project file:
```

```
Just like package references are resolved by ID, it would be cool to introduce a shorthand for project references. Instead of referencing a project by path, reference it by name:
```

```
The example above will try to resolve `Foo.csproj` or `Foo.fsproj` etc. from folders within the solution scope. This will allow one to specify a project reference by name instead of path.

What are your thoughts about this?

_Copied from original issue: dotnet/sdk#2432_

Start with the project-file ItemGroup and the existing ProjectReference behavior described in the issue. Investigate how references are resolved within the solution scope and how Foo.csproj or Foo.fsproj could be found from the shorthand name. Done means a documented, agreed design and working resolution by project name without a path.

Written by the indexing model from the issue text.