update namespacing options to reflect simpler structure

OpenEXR 3 simplifies it's library structure, but didn't simplify namespacing.

```cmake
set(OPENEXR_NAMESPACE_CUSTOM "0" CACHE STRING "Whether the namespace has been customized (so external users know)")
set(OPENEXR_INTERNAL_IMF_NAMESPACE "Imf_${OPENEXR_VERSION_API}" CACHE STRING "Real namespace for Imath that will end up in compiled symbols")
set(OPENEXR_IMF_NAMESPACE "Imf" CACHE STRING "Public namespace alias for OpenEXR")
set(OPENEXR_PACKAGE_NAME "OpenEXR ${OPENEXR_VERSION}" CACHE STRING "Public string / label for displaying package")

# Namespace-related settings, allows one to customize the
# namespace generated, and to version the namespaces
set(ILMBASE_NAMESPACE_CUSTOM "0" CACHE STRING "Whether the namespace has been customized (so external users know)")
set(ILMBASE_INTERNAL_IEX_NAMESPACE "Iex_${ILMBASE_VERSION_API}" CACHE STRING "Real namespace for Iex that will end up in compiled symbols")
set(ILMBASE_INTERNAL_ILMTHREAD_NAMESPACE "IlmThread_${ILMBASE_VERSION_API}" CACHE STRING "Real namespace for IlmThread that will end up in compiled symbols")
set(ILMBASE_IEX_NAMESPACE "Iex" CACHE STRING "Public namespace alias for Iex")
set(ILMBASE_ILMTHREAD_NAMESPACE "IlmThread" CACHE STRING "Public namespace alias for IlmThread")
set(ILMBASE_PACKAGE_NAME "IlmBase ${ILMBASE_VERSION}" CACHE STRING "Public string / label for displaying package")
```

Is there any reason not to strip it down to something like this, or is there an even simpler option? I seem to recall some recent discussion about simplifying the namespacing overall, in the c++ code.

```cmake
set(OPENEXR_NAMESPACE_CUSTOM "0" CACHE STRING "Whether the namespace has been customized (so external users know)")
set(OPENEXR_INTERNAL_IMF_NAMESPACE "Imf_${OPENEXR_VERSION_API}" CACHE STRING "Real namespace for Imath that will end up in compiled symbols")
set(OPENEXR_IMF_NAMESPACE "Imf" CACHE STRING "Public namespace alias for OpenEXR")
set(OPENEXR_PACKAGE_NAME "OpenEXR ${OPENEXR_VERSION}" CACHE STRING "Public string / label for displaying package")
```

Start by locating the CMake namespace settings shown in the issue and compare the OpenEXR block with the legacy IlmBase block. Review the related C++ namespacing discussion before deciding on the smallest supported configuration. Done means the namespace options are simplified consistently and the relevant CMake configuration still succeeds.

Written by the indexing model from the issue text.