Make it easier to express dependencies among projects without referencing output assembly

_From @tmat on October 8, 2018 23:44_

It is sometimes necessary to build project A before project B without project B referencing the assembly that A generates. In such cases one can use `ProjectReference` with `ReferenceOutputAssembly="false"`.

This however is not sufficient in all scenarios, especially when these projects target different, incompatible frameworks and/or multi-target.

Turns out 3 properties need to be set to make this work:
```xml

```

This is much more complex than it should be.

Proposal: introduce a new item that can be use to express that this project depends on building another project, but has no implication on references. Such item could be called e.g. `ProjectBuildOrderDependency`, `DependsOnProject`, etc.

```xml

```

_Copied from original issue: dotnet/sdk#2574_