Prebuilt report should detect "downloadDependencies" in project.assets.json

For example, this (PackageDownload):

```
"downloadDependencies": [
{
"name": "Microsoft.AspNetCore.App.Runtime.linux-musl-x64",
"version": "[5.0.0-preview.8.20414.8, 5.0.0-preview.8.20414.8]"
},
{
"name": "Microsoft.AspNetCore.App.Runtime.linux-x64",
"version": "[5.0.0-preview.8.20414.8, 5.0.0-preview.8.20414.8]"
},
...
{
"name": "Microsoft.NETCore.App.Runtime.linux-x64",
"version": "[5.0.0-preview.8.20407.11, 5.0.0-preview.8.20407.11]"
},
{
"name": "Microsoft.NETCore.App.Runtime.win-x64",
"version": "[5.0.0-preview.8.20407.11, 5.0.0-preview.8.20407.11]"
}
],
```

Doesn't show up here:

```xml



```

`grep`ping for `Microsoft.NETCore.App.Runtime.win-x64/5.0.0-preview.8.20407.11` doesn't quite work to find these, because they're not using the usual format. It's also not trivial to write a working `grep` because it's across multiple lines and uses a version range format rather than what was actually restored.

Something like `grep -i 'Microsoft.NETCore.App.Runtime.win-x64/5.0.0-preview.8.20407.11' -r src` does find you `project.nuget.cache` files, though, and you can open the sibling project.assets.json file to find the `downloadDependencies` node:

```
src/runtime.bf456654f9a4f9a86c15d9d50095ff29cde5f0a4/artifacts/obj/coreclr/crossgen2/project.nuget.cache: "/work/packages/restored/microsoft.netcore.app.runtime.win-x64/5.0.0-preview.8.20407.11/microsoft.netcore.app.runtime.win-x64.5.0.0-preview.8.20407.11.nupkg.sha512",
```

The prebuilt report should really include these.