tp: inline traceconv conversion into the shell subcommands (#6396)

The `convert` subcommand used to rebuild an argv and hand it to
TraceconvMain. Call the trace_to_text functions directly instead.

While doing this, split the symbolization modes out of `convert`, which
is
now only about turning a trace into another artifact:

  - bundle becomes a top-level subcommand.
  - symbolize and deobfuscate move under a new `util` subcommand.
  - profile and java_heap_profile stay in convert.

convert_helpers holds the stdin/stdout setup and the
text-proto-to-binary
path shared by convert and util. Output is unchanged from traceconv,
except
firefox and decompress_packets now exit 0 on success (they used to
return
the bool result directly, so success exited 1).

TraceconvMain and the standalone binary are removed later in the stack.