[UR] Support fetch adapter source individually

This patch and its counterpart in oneapi-src/unified-runtime#1410 add
CMake support for fetching an individual Unified Runtime adapter's
source code from a different repo/tag combination using the new
`fetch_adapter_source()` CMake function. Only the source is cloned, it
is not added to the build directly.

Instead, the path to the adapter source is passed into Unified Runtime
clone described by the `UNIFIED_RUNTIME_REPO` and `UNIFIED_RUNTIME_TAG`
CMake variables. This clone is the source of truth for the Unified
Runtime API and drives the build of the external adapter source.

Using `fetch_adapter_source()` is optional.

sycl/plugins/unified_runtime/CMakeLists.txt

@@ -56,14 +56,39 @@ endif()
 if(SYCL_PI_UR_USE_FETCH_CONTENT)
   include(FetchContent)
 
+  # The fetch_adapter_source function can be used to perform a separate content
+  # fetch for a UR adapter, this allows development of adapters to be decoupled
+  # from each other.
+  #
+  # Args:
+  #   * name - Must be the directory name of the adapter
+  #   * repo - A valid Git URL of a Unified Runtime repo
+  #   * tag - A valid Git branch/tag/commit in the Unified Runtime repo
+  function(fetch_adapter_source name repo tag)
+    message(STATUS "Will fetch Unified Runtime ${name} adapter from ${repo} at ${tag}")
+    set(fetch-name unified-runtime-${name})
+    FetchContent_Declare(${fetch-name}
+      GIT_REPOSITORY ${repo} GIT_TAG ${tag})
+    # We don't want to add this repo to the build, only fetch its source.
+    FetchContent_Populate(${fetch-name})
+    # Get the path to the source directory
+    string(TOUPPER ${name} NAME)
+    set(source_dir_var UR_ADAPTER_${NAME}_SOURCE_DIR)
+    FetchContent_GetProperties(${fetch-name} SOURCE_DIR UR_ADAPTER_${NAME}_SOURCE_DIR)
+    # Set the variable which informs UR where to get the adapter source from.
+    set(UR_ADAPTER_${NAME}_SOURCE_DIR
+      "${UR_ADAPTER_${NAME}_SOURCE_DIR}/source/adapters/${name}"
+      CACHE PATH "Path to external '${name}' adapter source dir" FORCE)
+  endfunction()
+
   set(UNIFIED_RUNTIME_REPO "https://github.com/oneapi-src/unified-runtime.git")
-  # commit 09be0881b727fadb1c04b38c00d2562d7dc6875f
-  # Merge: bb589ca8 e9f855d4
+  # commit 6513abc404979fa109d64500bf899e632d511291
+  # Merge: 09be0881 6d586094
   # Author: Kenneth Benzie (Benie) <k.benzie@codeplay.com>
-  # Date:   Thu Mar 14 22:10:28 2024 +0000
-  #     Merge pull request #1429 from nrspruit/l0_p2p_device_query
-  #     [L0] Support for urUsmP2PPeerAccessGetInfoExp to query p2p access info
-  set(UNIFIED_RUNTIME_TAG 09be0881b727fadb1c04b38c00d2562d7dc6875f)
+  # Date:   Thu Mar 14 22:38:53 2024 +0000
+  #     Merge pull request #1410 from kbenzie/benie/cmake-external-adapter-source-dirs
+  #     [CMake] Support external adapter source dirs
+  set(UNIFIED_RUNTIME_TAG 6513abc404979fa109d64500bf899e632d511291)
 
   if(SYCL_PI_UR_OVERRIDE_FETCH_CONTENT_REPO)
     set(UNIFIED_RUNTIME_REPO "${SYCL_PI_UR_OVERRIDE_FETCH_CONTENT_REPO}")