Add bi output group for index builds without linking

Introduce a new `bi` (build indexing) output group that contains only
indexstores and their filelist, without linked product binaries. Index
builds use this instead of `bp` to trigger transitive Swift compilation
(producing swiftmodules, indexstores, and generated headers) while
avoiding materialization of large linked binaries.

This reduces remote cache download volume and build time for index
builds, since only compilation artifacts needed for indexing are
requested.

Author: ra1028

xcodeproj/internal/bazel_integration_files/generate_index_build_bazel_dependencies.sh

@@ -10,11 +10,12 @@ readonly config="${BAZEL_CONFIG}_indexbuild"
 output_groups=(
   # Compile params
   "bc $BAZEL_TARGET_ID"
-  # Products (i.e. bundles) and index store data. The products themselves aren't
-  # used, they cause transitive files to be created. We use
-  # `--remote_download_regex` below to collect the files we care
+  # Indexstores and their filelist. Requesting indexstore directories triggers
+  # transitive Swift compilation (producing swiftmodules, indexstores, and
+  # generated headers) without linking, avoiding large product binaries. We use
+  # `--remote_download_regex` below to download the files we care
   # about.
-  "bp $BAZEL_TARGET_ID"
+  "bi $BAZEL_TARGET_ID"
 )
 
 indexstores_filelists=()

xcodeproj/internal/files/output_files.bzl

@@ -262,8 +262,17 @@ def _collect_output_files(
         transitive = [transitive_products],
     )
 
+    # Indexing output group: contains indexstores (to trigger Swift compilation)
+    # and the indexstores filelist (for import_indexstores). Unlike `bp`, this
+    # does not include linked products, avoiding large binary materialization.
+    indexing_depset = memory_efficient_depset(
+        [indexstores_filelist],
+        transitive = [transitive_indexstores],
+    )
+
     direct_group_list = [
         ("bc {}".format(id), transitive_compile_params),
+        ("bi {}".format(id), indexing_depset),
         ("bl {}".format(id), transitive_link_params),
         (products_output_group_name, products_depset),
     ]