Static constructor in SiteDefinition should not access IOC container

Description

Currently, the static constructor in SiteDefintion accesses the IOC container through usage of MakeReadOnly of SiteDefintion.Empty.
This can cause a problem under certain circumstances. For example, the first access to the type (which causes the static constructor) is done in a stack frame that is already dependent on SiteDefintion.Current (directly or indirectly). This is a problem because a circular dependency exception may result.