FindOpenGL: Provide imported targets for GL and GLU
Create OpenGL::GL and OpenGL::GLU imported targets using the locations found.
This feature was originally added by commit b7804cb6 but had to be reverted by commit 02e34de2 due to issue #15267 (closed). Since then we added support for IMPORTED_LIBNAME
to interface libraries, so use it to handle the case where we have only the library name without an absolute path.
Closes: #15267 (closed)