Project-system: Up to date check looks for all files in the project root, not at their own paths

[gulbanana]

Repro: https://github.com/gulbanana/repro-netcore-up2date-subdir

Problem: subdirectory\TextFile1.txt is PreserveNewest, so it should be up to date after a build. However, logging reports otherwise:

1>FastUpToDate: Latest write timestamp on input is 16/08/2017 11:20:10 AM on 'C:\code\repro-netcore-up2date-subdir\ClassLibrary1\ClassLibrary1.csproj'. (ClassLibrary1)
1>FastUpToDate: Earliest write timestamp on output is 16/08/2017 11:20:15 AM on 'C:\code\repro-netcore-up2date-subdir\ClassLibrary1\obj\Debug\netcoreapp2.0\ClassLibrary1.pdb'. (ClassLibrary1)
1>FastUpToDate: Checking PreserveNewest file 'C:\code\repro-netcore-up2date-subdir\ClassLibrary1\subdirectory\TextFile1.txt': (ClassLibrary1)
1>FastUpToDate:     Write 16/08/2017 11:14:25 AM: 'C:\code\repro-netcore-up2date-subdir\ClassLibrary1\subdirectory\TextFile1.txt'. (ClassLibrary1)
1>FastUpToDate:     Output file 'bin\Debug\netcoreapp2.0\TextFile1.txt' does not exist. (ClassLibrary1)
1>FastUpToDate: Project is not up to date. (ClassLibrary1)

bin\Debug\netcoreapp2.0\subdirectory\TextFile1.txt exists, but something is looking at bin\Debug\netcoreapp2.0\TextFile1.txt instead and concluding that the file needs to be recreated.

[gulbanana]

this seems to happen for Content items but not for None items

[davkean]

Have a fix for this.

[davkean]

This is because we're not rooting the output path correctly.

[davkean]

This has been resolved and will be fixed in the next update of Visual Studio 15.4.