[FIX] Ensure hnswlib can be found from RAFT's build dir (#2145)

This PR fixes using RAFT from a build dir, i.e. `cmake -S cuml/cpp -B cuml/cpp/build -Draft_ROOT=raft/cpp/build`.

Without this fix, CMake errors when cuML does `find_package(RAFT)` with the following error:
```
CMake Error at raft/cpp/build/latest/raft-targets.cmake:56 (set_target_properties):
  The link interface of target "raft::raft" contains:

    hnswlib::hnswlib

  but the target was not found.  Possible reasons include:


    * There is a typo in the target name.
    * A find_package call is missing for an IMPORTED target.
    * An ALIAS target is missing.
```

Authors:
  - Paul Taylor (https://github.com/trxcllnt)

Approvers:
  - Corey J. Nolet (https://github.com/cjnolet)
  - Bradley Dice (https://github.com/bdice)
  - Vyas Ramasubramani (https://github.com/vyasr)

URL: #2145

cpp/cmake/thirdparty/get_hnswlib.cmake

@@ -50,20 +50,30 @@ function(find_and_configure_hnswlib)
       install(TARGETS hnswlib EXPORT hnswlib-exports)
       install(DIRECTORY "${hnswlib_SOURCE_DIR}/hnswlib/" DESTINATION include/hnswlib)
 
-      # write export rules
-      rapids_export(
-        BUILD hnswlib
-        VERSION ${PKG_VERSION}
-        EXPORT_SET hnswlib-exports
-        GLOBAL_TARGETS hnswlib
-        NAMESPACE hnswlib::)
+      # write install export rules
       rapids_export(
         INSTALL hnswlib
         VERSION ${PKG_VERSION}
         EXPORT_SET hnswlib-exports
         GLOBAL_TARGETS hnswlib
         NAMESPACE hnswlib::)
     endif()
+
+    # write build export rules
+    rapids_export(
+      BUILD hnswlib
+      VERSION ${PKG_VERSION}
+      EXPORT_SET hnswlib-exports
+      GLOBAL_TARGETS hnswlib
+      NAMESPACE hnswlib::)
+
+    include("${rapids-cmake-dir}/export/find_package_root.cmake")
+
+    # When using RAFT from the build dir, ensure hnswlib is also found in RAFT's build dir. This
+    # line adds `set(hnswlib_ROOT "${CMAKE_CURRENT_LIST_DIR}")` to build/raft-dependencies.cmake
+    rapids_export_find_package_root(
+      BUILD hnswlib [=[${CMAKE_CURRENT_LIST_DIR}]=] EXPORT_SET raft-exports
+    )
   endif()
 endfunction()