This is added so that some texture pipeline with point light and spot
light could work as before. Some people use the Normal socket from
Texture Coordinate node for texturing light, however the Normal there is
actually the incoming light direction and should be corrected. Using the
Parametric socket from Geometry node + normal transform from world to
object with Vector Transform node delivers the same result as using the
Normal socket from Texture Coordinate node.
Currently for lights only normal transformation works, because only
there we fetch light transform properly. This is a confusing behaviour,
but testing if it's a lamp in all relevant functions could have bad
impact on the performance. A more proper solution would be to change
lights to real objects, which is planned for the future.

Pull Request: https://projects.blender.org/blender/blender/pulls/108666
* Replace license text in headers with SPDX identifiers.
* Remove specific license info from outdated readme.txt, instead leave details
to the source files.
* Add list of SPDX license identifiers used, and corresponding license texts.
* Update copyright dates while we're at it.
Ref D14069, T95597
Remove prefix of filenames that is the same as the folder name. This used
to help when #includes were using individual files, but now they are always
relative to the cycles root directory and so the prefixes are redundant.
For patches and branches, git merge and rebase should be able to detect the
renames and move over code to the right file.