I2I: Consolidate source code under `src/`

In the earlier days of amp, the root directory was fairly sparse. "Top-level" directories like `3p`, `ads`, `builtins`, and `extensions` had separate functions, while `src` was synonymous with the AMP runtime. IIUC, one motivation for this at the time was so external contributors could easily view the repo and understand where to contribute an ad provider, 3p integration, or new component/extension.

Today, the root directory has dozens of config files and directories (`.babel-cache/`, `.circleci/`, `.codecov.yml`, `.css-cache/`, `.editorconfig`, `.eslintignore`, `.eslintplugin.js`, `.eslintrc.js`, `.git/`, `.gitattributes`, `.github/`, `.gitignore`, `.lando.yml`, `.lgtm.yml`, `.npmrc`, `.nyc_output/`, `.pre-closure-cache/`, `.prettierignore`, `.prettierrc`, `.renovaterc.json`, `.vscode/`), build/dist files, `node_modules`, `third_party`, and a handful of other directories containing tools, docs, and more.

More generally, I suspect if we re-created the repo today, we'd put all the source files under `src`, separate from built system, build files, documentation, and testing. Down the line, this can also allow for consolidation and simplification of some lint rules, presubmits, etc.

Start by inventorying the root-level source directories named in the issue, including `3p`, `ads`, `builtins`, `extensions`, and `src`, alongside the listed build and presubmit configuration files. Define the target layout and the required build, lint, presubmit, and contributor workflows before attempting consolidation; the issue does not specify tests or a completion checklist.

Written by the indexing model from the issue text.