What does your PR fix?

This PR transforms port openexr into a port openexr2 which can coexist with a new port openexr3. This approach is meant to mitigate the openexr update issues which turned up in previous attempts to update openexr. The availability of OpenEXR 3 is urgent to unlock further updates to consuming ports and to restore broken features such as openimageio[opencolorio] (cf. [openimageio] Build error #26089).
Notes:

• Includes are moved to include/openexr2 / include/openexr3.
• Pkgconfig files can't coexist under the same name. openexr2's main pc file is renamed to OpenEXR2.pc.
• There can only be one CMake wrapper for package name OpenEXR. However, there shouldn't be a wrapper for OpenEXR CONFIG. That's why the wrapper is maintained in legacy port openexr2.
• CMake config selection (between different versions) can be left to CMake. However, it needs to be explicitly instructed to use "natural descending" search order. This is done in the wrapper. (Selection is not needed if only openexr3 is installed.)
• Tested locally with port openimageio[opencolorio].

Previous work: [openexr3] Add new port #25238 (only adding openexr3), [openexr] Update to 3.1.5 #24249, [openexr] Update to 3.1.4 #20957 (direct updates of openexr).

Which triplets are supported/not supported? Have you updated the CI baseline?

unchanged, no

Does your PR follow the maintainer guide?

WIP

If you have added/updated a port: Have you run ./vcpkg x-add-version --all and committed the result?

I am still working on this PR.