Move content file watching from Roslyn to Project System

The Project System is responsible for tracking all outputs of a (design-time) build, which includes the content of the files generated by various targets to `obj` directory that are part of the compilation.

In a cloud environment Roslyn services might run on a different machine than DTB and thus can't watch for file changes made during DTB. We need the Project System to notify Roslyn of all DTB outputs that affect the compilation.

Note that this also affects fully local scenarios, such as Edit and Continue in WPF projects. Currently the Roslyn file watchers detect changes made by WPF single file generators during build after the debugging session have started. See https://dev.azure.com/devdiv/DevDiv/_workitems/edit/970821