Split Targets and Config and add find_package

[hjmallon]

This last CMake update splits out the XConfig.cmake and the XTargets.cmake files so that find_dependency lines can be added for the OpenEXRConfig.cmake. The process is described in docs below.

• https://cmake.org/cmake/help/latest/module/CMakeFindDependencyMacro.html#module:CMakeFindDependencyMacro
• https://cmake.org/cmake/help/latest/module/CMakePackageConfigHelpers.html?highlight=package_init#command:write_basic_package_version_file.

[hjmallon]

Rebased on master.

[meshula]

lgtm

[cary-ilm]

We discussed this in the steering committee. This is almost certainly a result of the complication of the top-level "super-build" setup that builds the submodules IlmBase, OpenEXR, PyIlmBase, OpenEXR_Viewers. Our intention with the 3.0 release later in the year is to split these projects into separate github repos, creating proper dependencies between them, which will mean an entire re-vamp of the cmake setup. At this point, this need for a special find_package will change drastically or go away entirely. We'd like to minimize any investment in maintaining the existing setup. This PR adds a level of additional complexity that we'd like to avoid.

[cary-ilm]

As mentioned above, this entire structure is slated to be retired soon, but this change looks fine for now, so merging it in.