Fix issue with long path names.

In deeply nested workspace and/or long target names the paths for the
various DWYU artifacts can become quite long. At some point they are too
large for Bazel and the OS to handle and consequently DWYU fails.

We fix this by:
- We use only the name of the to be analyzed target instead of its full
  label as prefix for the DWYU artifacts. The artifacts are generated inside
  bazel-out in the package location of the to be analyzed target and thus
  the name is sufficient as suffix to guarantee unique results.
- For the DWYU artifacts depending on dependencies we no longer use the
  dependency label as path suffix, but instead use the hash of the label.
  This gives us an upper bound on the max length of the generated file names
  while still making name collision quite unlikely. That the names are less
  verbose does not matter. If one wants to debug, the name of the dependency
  is contained in the file either way.

src/aspect/dwyu.bzl

@@ -1,9 +1,6 @@
 load("@bazel_tools//tools/build_defs/cc:action_names.bzl", "CPP_COMPILE_ACTION_NAME")
 load("@bazel_tools//tools/cpp:toolchain_utils.bzl", "find_cpp_toolchain")
 
-def _label_to_name(label):
-    return str(label).replace("@", "").replace("//", "").replace("/", "_").replace(":", "_")
-
 def _get_target_sources(rule):
     public_files = []
     private_files = []
@@ -57,7 +54,7 @@ def _process_dependencies(ctx, target, deps, verbose):
         ctx,
         target = dep,
         defines = [],
-        output_path = "{}_processed_dep_{}.json".format(_label_to_name(target.label), _label_to_name(dep.label)),
+        output_path = "{}_processed_dep_{}.json".format(target.label.name, hash(str(dep.label))),
         is_target_under_inspection = False,
         verbose = verbose,
     ) for dep in deps]
@@ -160,7 +157,7 @@ def dwyu_aspect_impl(target, ctx):
         ctx,
         target = target,
         defines = _gather_defines(ctx, target_compilation_context = target[CcInfo].compilation_context),
-        output_path = "{}_processed_target_under_inspection.json".format(_label_to_name(target.label)),
+        output_path = "{}_processed_target_under_inspection.json".format(target.label.name),
         is_target_under_inspection = True,
         verbose = False,
     )
@@ -175,7 +172,7 @@ def dwyu_aspect_impl(target, ctx):
         verbose = False,
     )
 
-    report_file = ctx.actions.declare_file("{}_dwyu_report.json".format(_label_to_name(target.label)))
+    report_file = ctx.actions.declare_file("{}_dwyu_report.json".format(target.label.name))
     args = ctx.actions.args()
     args.add("--report", report_file)
     args.add_all("--public_files", public_files, expand_directories = True, omit_if_empty = False)