[rust] Split the `rust_logger` crate into smaller modules.

This helps to hide some implementation details as private items of
the new, smaller modules.  In addition to moving the new modules into
`//base/logging/rust_logger` subdirectory, this CL also moves tests
into the same directory.  Summary of the changes/moves:

* `base/logging/logger.rs` has been split into 4 modules:
    - `base/logging/rust_logger/lib.rs` (entry point via C API)
    - `base/logging/rust_logger/log_crate_integration.rs` and
      `base/logging/rust_logger/custom_panic_hook.rs`
    - `base/logging/rust_logger/print_rust_log.rs` (low-level
      implementation for invoking `LOG(...)` from Rust).
* `base/logging/rust_log_integration.h/.cc` has been moved to
  `base/logging/rust_logger/print_rust_log_ffi.h/.cc`
* `base/test/logging/test_rust_logger_consumer.rs` has been moved to
  `base/logging/rust_logger/test_support.rs`
* `base/logging/rust_log_integration_unittest.cc` has been moved to
  `base/logging/rust_logger/unittests.cc`

Bug: 495537792
Change-Id: I0d79a0a7dd53aac63978caf54bb6e538406d00a4
Bypass-Check-License: New files were moved or forked from older files.
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7696722
Auto-Submit: Łukasz Anforowicz <lukasza@chromium.org>
Reviewed-by: Stefano Duo <stefanoduo@google.com>
Commit-Queue: Łukasz Anforowicz <lukasza@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1611072}