Circular dependency error should include more information.

When this throws, the error is fairly inscrutable:

https://github.com/Microsoft/msbuild/blob/951b2f068956429acf8ad283034f1031bbcf80f0/src/Build/BackEnd/Components/RequestBuilder/TargetBuilder.cs#L702-L714

It just says

```
error MSB4006: There is a circular dependency in the target dependency graph involving target "PrepareProjectReferences".
```

That's not very easy to find and fix.

We could keep a list of parents so that we could say something like

```
There is a circular dependency in the target dependency graph involving target {targetSpecification.TargetName}. Target {parentTargetEntry} has a {buildReason} dependency on it, but it is depended upon by {string.Join("<-", listOfParents)}.
```

Maybe only at higher verbosities.