Deprecate Project.this(PackageManager, NativePath)

This overload was originally only used from Dub.loadPackage(NativePath)
before it was changed (although the Project should only ever be instantiated
from the Dub class). But it is doing IO and its soft failure approach
is a misfeature, hence we deprecated it.

source/dub/project.d

@@ -57,6 +57,7 @@ class Project {
 			project_path = Path of the root package to load
 			pack = An existing `Package` instance to use as the root package
 	*/
+	deprecated("Load the package using `PackageManager.getOrLoadPackage` then call the `(PackageManager, Package)` overload")
 	this(PackageManager package_manager, NativePath project_path)
 	{
 		Package pack;