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.