wrapper: add exemplar target files instead of dirs for @@ldpaths@@

Instead of forming LD_LIBRARY_PATH from the dirname of shared libs of
our deps, put in the actual shared library path names, instead. Then,
in the wrapper, when building up LD_LIBRARY_PATH, use bash $(dirname)
to get just the directory component from the resolved $(rlocation) of
the shared lib.

If multiple shared lib deps exist in the same runfiles directory,
it'll just take a single one to avoid repetition in LD_LIBRARY_PATH.

Testing:

    bazel test use_toolchain_test
    bazel run tool:wrapped_tool /dev/stdout $PWD/foo.txt

tool/wrapper.bzl

@@ -37,7 +37,7 @@ def _tool_wrapper_impl(ctx):
 
         for path in file_paths:
             if path.endswith(".so") or ".so." in path:
-                dirs[paths.dirname(path)] = True
+                dirs.setdefault(paths.dirname(path), []).append(path)
 
     exec_dep_files, exec_path = _resolve_dep_paths(ctx, ctx.attr.executable)
     transitive_runfiles.append(exec_dep_files)
@@ -47,7 +47,7 @@ def _tool_wrapper_impl(ctx):
         output = ctx.outputs.out,
         substitutions = {
             "@@run@@": exec_path[0],
-            "@@ldpaths@@": shell.array_literal(list(dirs)),
+            "@@ldpaths@@": shell.array_literal([k[0] for k in dirs.values()]),
         },
         is_executable = True,
     )

tool/wrapper.sh.tmpl

@@ -30,7 +30,7 @@ do
    path=$(rlocation "$p")
    if [ -n "$path" ]
    then
-      _add "$path"
+      _add "$(dirname "$path")"
    fi
 done
 _add "$orig"