Shorten protoc module names

Perfetto comes with a lot of generated proto headers. If we don't do
this, we will end up with lots of generated headers with long module
names (because they are all nested under
//third_party/perfetto/protos/perfetto), which bubble up and combine
with each other along the build graph, leading to huge
`generated_headers` lists that then blow up the build.

For example, this turns:

  tot_cronet_third_party_perfetto_protos_perfetto_config_process_stats_zero_gen__testing_headers

Into:

  tot_cronet_1baaa48e_process_stats_zero_gen__testing_h

Bug: b:346770535
Change-Id: Ic6e7479328ae80327a7e541ac874b307d4e40358
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6433671
Auto-Submit: Etienne Dechamps <edechamps@google.com>
Commit-Queue: Etienne Dechamps <edechamps@google.com>
Reviewed-by: Mohannad Farrag <aymanm@google.com>
Cr-Commit-Position: refs/heads/main@{#1443377}