[roll] Roll fuchsia [bazel_sdk] Rename FuchsiaDebugSymbolInfo.build_id_dirs for clarity.

Rename the FuchsiaDebugSymbolInfo 'build_id_dirs' key to
'build_id_dirs_mapping' to clarify that this is a dictionary, as the
same name is used for lists of File values or depsets in other parts
of the build rules.

Also change the name of 'source_dir' to 'source_search_root' as it
is not always a directory, and clarify its meaning in the documentation.

+ Rename collect_debug_symbols() to merge_debug_symbol_infos() as this
  better describes what the function does.

Note that using //:BUILD.bazel in @fuchsia_clang//:debug_symbols and
@fuchsia_sdk//:debug_symbols is properly invalid, but this may be
addressed in a different CL. I.e.:

- For @fuchsia_clang//:debug_symbols, manual inspection of the
  binaries provided with the toolchain show that the source paths
  are either relative to the sysroot include directory (e.g. "stdlib.h")
  or absolute paths from the original build bot that generated the
  prebuilt binaries (e.g.
  "/b/s/w/ir/x/w/llvm_build/tools/clang/stage2-bins/include/c++/v1/__utility.copy.h").

  In practice, there is little chance these can be found properly
  when developing in-tree or with the Fuchsia SDK. So whatever
  value should be ok.

- For @fuchsia_sdk//:debug_symbols, the .build-id/ repository is now
  empty by default, since individual prebuilts are exposed using
  fuchsia_unstripped_binary() targets that does not provide a
  "source_search_root" value, which will then default to
  BAZEL_WORKSPACE_DIRECTORY, i.e.
  $NINJA_BUILD_DIR/gen/build/bazel/workspace/

  However, the source paths embedded in the binaries are relative
  to $NINJA_BUILD_DIR (e.g. "../../sdk/lib/fidl/cpp/wire/client_base.cc"),
  it may makes sense, for in-tree development, to change this
  value to a correct path. For OOT development, that should not
  matter.

Original-Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/1249584
Original-Revision: f58b4cae0b4ce87f81c12c91dd5867ee67bb1945
GitOrigin-RevId: ee90f56defcc951782a49b6574032ee200d57a7a
Change-Id: Iaecf0733af5351ea8ea260eb3b29a9b1dd8f75a4