Define behavior of PathUtils.ResolveAppRelative and variants

ResolveAppRelative and ResolveAppRelativeAssumeAppRelative currently have undefined behavior in the absence of an HttpEnvironment. In the develop branch, one resolves to a physical path, while the other remains a virtual path.

Start by inspecting PathUtils.ResolveAppRelative and PathUtils.ResolveAppRelativeAssumeAppRelative and compare their behavior in the develop branch when no HttpEnvironment is present. Review callers to determine whether the result should be physical or virtual, then define the expected behavior consistently for both variants.

Written by the indexing model from the issue text.