[cppyy] Move libcppyy.so to <LIBDIR>/cppyy/libcppyy.so

When compiling ROOT in Gentoo, the following warning was issued:
 * Verifying compiled files for python3.13
 *
 * QA Notice: The following unexpected files/directories were found
 * top-level in the site-packages directory:
 *
 *   /usr/lib/python3.13/site-packages/libcppyy.so.6.37.01
 *
 * This is most likely a bug in the build system.  More information
 * can be found in the Python Guide:
 * https://projects.gentoo.org/python/guide/qawarn.html#stray-top-level-files-in-site-packages

Similarly to #14917, this can be solved by moving it to <LIBDIR>/cppyy.

Therefore:
- Move the libray
- Update the library's RUNPATHs for it to find ROOT libraries.
- Use relative import paths to simplify finding libcppyy

Fix #20015.

Author: hageboeck

bindings/pyroot/cppyy/CPyCppyy/CMakeLists.txt

@@ -73,7 +73,8 @@ endif()
 add_library(cppyy SHARED src/CPyCppyyPyModule.cxx)
 
 # Set the suffix to '.so' and the prefix to 'lib'
-set_target_properties(cppyy PROPERTIES  ${ROOT_LIBRARY_PROPERTIES})
+set_target_properties(cppyy PROPERTIES  ${ROOT_LIBRARY_PROPERTIES}
+  LIBRARY_OUTPUT_DIRECTORY ${localruntimedir}/cppyy)
 if(MSVC)
   target_link_libraries(cppyy PRIVATE CPyCppyy)
   set_target_properties(cppyy PROPERTIES WINDOWS_EXPORT_ALL_SYMBOLS TRUE)
@@ -108,7 +109,7 @@ set_property(GLOBAL APPEND PROPERTY ROOT_EXPORTED_TARGETS cppyy)
 
 if(NOT MSVC)
   # Make sure that relative RUNPATH to main ROOT libraries is always correct.
-  ROOT_APPEND_LIBDIR_TO_INSTALL_RPATH(cppyy ${CMAKE_INSTALL_PYTHONDIR})
+  ROOT_APPEND_LIBDIR_TO_INSTALL_RPATH(cppyy ${CMAKE_INSTALL_PYTHONDIR}/cppyy)
   ROOT_APPEND_LIBDIR_TO_INSTALL_RPATH(CPyCppyy ${CMAKE_INSTALL_LIBDIR})
 endif()
 
@@ -119,9 +120,9 @@ install(TARGETS CPyCppyy EXPORT ${CMAKE_PROJECT_NAME}Exports
                             ARCHIVE DESTINATION ${CMAKE_INSTALL_LIBDIR} COMPONENT libraries)
 
 install(TARGETS cppyy EXPORT ${CMAKE_PROJECT_NAME}Exports
-                            RUNTIME DESTINATION ${CMAKE_INSTALL_PYTHONDIR} COMPONENT libraries
-                            LIBRARY DESTINATION ${CMAKE_INSTALL_PYTHONDIR} COMPONENT libraries
-                            ARCHIVE DESTINATION ${CMAKE_INSTALL_PYTHONDIR} COMPONENT libraries)
+                            RUNTIME DESTINATION ${CMAKE_INSTALL_PYTHONDIR}/cppyy COMPONENT libraries # Windows
+                            LIBRARY DESTINATION ${CMAKE_INSTALL_PYTHONDIR}/cppyy COMPONENT libraries
+                            ARCHIVE DESTINATION ${CMAKE_INSTALL_PYTHONDIR}/cppyy COMPONENT libraries)
 
 file(COPY ${headers} DESTINATION ${CMAKE_BINARY_DIR}/include/CPyCppyy)
 

bindings/pyroot/cppyy/cppyy/python/cppyy/__init__.py

@@ -339,7 +339,9 @@ def add_library_path(path):
 
         apipath_extra = os.path.join(os.path.dirname(apipath), 'site', 'python'+ldversion)
         if not os.path.exists(os.path.join(apipath_extra, 'CPyCppyy')):
-            import glob, libcppyy
+            import glob
+
+            import cppyy.libcppyy as libcppyy
             ape = os.path.dirname(libcppyy.__file__)
           # a "normal" structure finds the include directory up to 3 levels up,
           # ie. dropping lib/pythonx.y[md]/site-packages

bindings/pyroot/cppyy/cppyy/python/cppyy/_cpython_cppyy.py

@@ -27,7 +27,7 @@
 except ImportError:
     c = None
 
-import libcppyy as _backend
+import cppyy.libcppyy as _backend
 
 if c is not None:
     _backend._cpp_backend = c