refactor: consolidate `_bazel.bzl` and remove semantics.bzl

This issue is to track work related to consolidating and cleaning up the code now that it's been moved out of Bazel itself. The main reason for this is to simplify the implementation. When the code was in Bazel itself, having separate files for Bazel and Google-internal pieces sense because carrying patches for the code was particularly hard. However, this ended up making the code somewhat complicated: (1) a "semantics" plugin object with quite a few "hooks" into various low-level steps and (2) the "hook points" have turned into a brittle API that make it hard to communicate state from one step to another.

Most low-hanging fruit has been cleaned up, the three main things remaining are:

1. [x] Merging `py_executable_bazel.bzl` into `py_executable.bzl`
2. [x] Merging `common_bazel.bzl` into `common.bzl`
3. [x] Removing `semantics.bzl`
4. [ ] Removing "binary semantics" and "library semantics"

Cleaning this up will probably expose some natural points of simplification in how things are implemented.

issue のチェックリストと、残っているバイナリセマンティクスおよびライブラリセマンティクスの作業を確認してください。次に、参照されている `_bazel.bzl`、`semantics.bzl`、`py_executable.bzl`、`common.bzl` ファイルを調べ、完了した merges の後に何が残っているかを把握してください。これらのセマンティクス層を削除し、影響を受ける実装を簡素化したうえで、追跡対象のクリーンアップを未完了のまま残さなければ完了です。

索引モデルが issue の本文から書いたものです。