`PhysicalFilesWatcher` and `PhysicalFileProvider` should implement `IDisposable`

[dougbu]

PhysicalFilesWatcher holds system resources in its FileSystemWatcher field. But PhysicalFilesWatcher and the (public) PhysicalFileProvider class containing the watcher do not implement IDisposable. System resources are therefore not released until finalization.

[muratg]

@dougbu Was this a CR find, or were you investigating some leak?

[dougbu]

Found this when investigating file system limits hit (aka FileSystemWatcher leaks) doing functional testing w/ DNX Core on Linux. We realized our use of TestServer was a mess for other reasons and moved those tests to use an xUnit fixture. That minimized our TestServer count enough for testing in this environment. But not all PhysicalFileProvider instances are pinned by the server so customers could hit similar issues.

@Tratcher and I discussed the issue back then and I finally got around to filing it.