Deprecate string-as-version PackageManager.getBestPackage overload

Because using a typed Version or VersionRange is better than using
a string and we want to encourage version validation to be done
earlier than later.

source/dub/packagemanager.d

@@ -440,6 +440,7 @@ class PackageManager {
 	}
 
 	/// Ditto
+	deprecated("Use the overload that accepts a `Version` or a `VersionRange`")
 	Package getBestPackage(string name, string range)
 	{
 		return this.getBestPackage(name, VersionRange.fromString(range));