feat: support installing imported targets

[aminya]

Imported targets that include prebuilt libraries cannot be installed using package_project now. This tries to fix the issue

How to test

Try to import an externally built library and re-export it

add_library(built_by_bazel SHARED IMPORTED)
set_property(TARGET built_by_bazel PROPERTY IMPORTED_LOCATION "./bazel-bin/libbuilt_by_bazel.so")
set_target_properties(built_by_bazel PROPERTIES LINKER_LANGUAGE CXX)

target_link_libraries(built_by_bazel INTERFACE project_options project_warnings) # link project_options/warnings

set(INCLUDE_DIR "include") # must be relative paths
target_include_directories(
  built_by_bazel INTERFACE "$<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/${INCLUDE_DIR}>"
                             "$<INSTALL_INTERFACE:./${CMAKE_INSTALL_INCLUDEDIR}>"
)

# Package the project
package_project(
  TARGETS
  built_by_bazel
  PUBLIC_INCLUDES
  ${INCLUDE_DIR}
)

TODO

When the _filtered_list_targets is empty, nothing will be exported, which results in an error. In this case, the imported targets should be exported too.