While generating the restore graph file add the location to where the package dependency version is specified

### Explanation

I am using `Directory.Build.props` files to define package references and declare version variables (e.g. `4.9.0`) and reference this version variable in a Project.csproj (like ``).

It would be nice to include the location to the specific `.props` file in the restore graph from where the package was referenced or the version was specified. This enables other tools to update package version for projects that are specifing package versions through `.props` files.

Test Project file
```xml

netcoreapp2.1

runtime; build; native; contentfiles; analyzers
all

```

Solution directory contents:
```
/
- build/
- dependencies.props
- sources.props
- src/
- Project1/
- Project1.csproj
- Program.cs
- Project2/
- Project2.csproj
- Program.cs
- Directory.Build.props
- test/
- Project1.Tests/
- Project1.Tests.csproj
- Test.cs
- Project2.Tests/
- Project2.Tests.csproj
- Test.cs
- Directory.Build.props
- Directory.Build.props
- Directory.Build.targets
- Project.sln
- NuGet.config
- version.props
```

Command line
```
msbuild PROJECT_PATH /t:GenerateRestoreGraphFile /p:RestoreGraphOutputPath=TEMP_FILE_LOCATION
```

### Environment data
`msbuild /version` output:

Microsoft (R)-Buildmodul, Version 4.7.3056.0
[Microsoft .NET Framework, Version 4.0.30319.42000]
Copyright (C) Microsoft Corporation. Alle Rechte vorbehalten.

4.7.3056.0

OS info:
Edition: Windows 10 Pro
Version: 1803
Build: 17134.285

dotnet --info
.NET Core SDK (reflecting any global.json):
Version: 2.1.402
Commit: 3599f217f4

Runtime Environment:
OS Name: Windows
OS Version: 10.0.17134
OS Platform: Windows
RID: win10-x64
Base Path: C:\Program Files\dotnet\sdk\2.1.402\

Host (useful for support):
Version: 2.1.4
Commit: 85255dde3e

.NET Core SDKs installed:
1.0.4 [C:\Program Files\dotnet\sdk]
1.1.10 [C:\Program Files\dotnet\sdk]
2.0.0 [C:\Program Files\dotnet\sdk]
2.0.2 [C:\Program Files\dotnet\sdk]
2.1.2 [C:\Program Files\dotnet\sdk]
2.1.104 [C:\Program Files\dotnet\sdk]
2.1.105 [C:\Program Files\dotnet\sdk]
2.1.201 [C:\Program Files\dotnet\sdk]
2.1.202 [C:\Program Files\dotnet\sdk]
2.1.300-preview2-008533 [C:\Program Files\dotnet\sdk]
2.1.300-rc1-008673 [C:\Program Files\dotnet\sdk]
2.1.300 [C:\Program Files\dotnet\sdk]
2.1.301 [C:\Program Files\dotnet\sdk]
2.1.302 [C:\Program Files\dotnet\sdk]
2.1.400 [C:\Program Files\dotnet\sdk]
2.1.401 [C:\Program Files\dotnet\sdk]
2.1.402 [C:\Program Files\dotnet\sdk]

.NET Core runtimes installed:
Microsoft.AspNetCore.All 2.1.0-preview2-final [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.All]
Microsoft.AspNetCore.All 2.1.0-rc1-final [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.All]
Microsoft.AspNetCore.All 2.1.0 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.All]
Microsoft.AspNetCore.All 2.1.1 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.All]
Microsoft.AspNetCore.All 2.1.2 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.All]
Microsoft.AspNetCore.All 2.1.3 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.All]
Microsoft.AspNetCore.All 2.1.4 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.All]
Microsoft.AspNetCore.App 2.1.0-preview2-final [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 2.1.0-rc1-final [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 2.1.0 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 2.1.1 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 2.1.2 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 2.1.3 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 2.1.4 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.NETCore.App 1.0.5 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 1.0.12 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 1.1.2 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 1.1.9 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 2.0.0 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 2.0.3 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 2.0.6 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 2.0.7 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 2.0.9 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 2.1.0-preview2-26406-04 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 2.1.0-rc1 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 2.1.0 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 2.1.1 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 2.1.2 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 2.1.3 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 2.1.4 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]