Merge all runfiles of @bazel_tools//tools/bash/runfiles (#3292)

As `@bazel_tools//tools/bash/runfiles` is now an alias to
`@rules_shell//shell/runfiles` with Bazel@HEAD, consumers must properly
merge all its runfiles rather than assuming that the `sh_library` target
just consists of a single file.

Work towards
bazelbuild/examples#557 (comment)

Author: fmeum

crate_universe/private/crates_vendor.bzl

@@ -469,15 +469,13 @@ def _crates_vendor_impl(ctx):
         is_executable = True,
     )
 
+    runfiles = ctx.runfiles(files = cargo_bazel_runfiles, transitive_files = toolchain.all_files)
     if runner.basename.endswith(".sh"):
-        cargo_bazel_runfiles.append(ctx.file._bash_runfiles)
+        runfiles = runfiles.merge(ctx.attr._bash_runfiles[DefaultInfo].default_runfiles)
 
     return DefaultInfo(
         files = depset([runner]),
-        runfiles = ctx.runfiles(
-            files = cargo_bazel_runfiles,
-            transitive_files = toolchain.all_files,
-        ),
+        runfiles = runfiles,
         executable = runner,
     )
 
@@ -579,7 +577,6 @@ CRATES_VENDOR_ATTRS = {
     "_bash_runfiles": attr.label(
         doc = "The runfiles library for bash.",
         cfg = "target",
-        allow_single_file = True,
         default = Label("@bazel_tools//tools/bash/runfiles"),
     ),
 }