Fix incorrect TPA error

Fix incorrect TPA error

The isManagedAssembly() function receives a file name as an input and checks only the extension.
But, it calls isR2RImage(), which requires a full path to the file internally.

Upon checking the usage of the isManageAssembly() function,
it is confirmed that it is used only to check whether it is a dll file or not.

So, the isManagedAssembly() function has been modified to return true even in the case of including a native image,
and related codes have been reorganized.