🏗🚀 Speed up pre-closure babel transforms

[rsimha]

Background:

• AMP's minified build (gulp dist) and type checker (gulp check-types) run a set of babel transforms on source files before invoking closure compiler.
• Today, transforms are run on all source files at once, after which the results are written to disk in a temp directory. Following this, closure compiler is invoked on the temp directory.
• This is particularly inefficient for flags like --core_runtime_only and --noextensions, which only compile a small subset of source files.

This PR does the following:

• Refactors pre-closure babel transforms to run directly on the gulp.src stream
• Moves all transform code to build-system/compile/pre-closure-babel.js
• Implements a caching mechanism so every source file is transformed at most once
• Deletes transferSrcsToTempDir() and all associated code
• Ensures that only those source files that need to be compiled are transformed

Performance improvements (Macbook Pro):

Command	Before	After
gulp dist --core_runtime_only	~50s	~30s
gulp check-types	~1m 40s	~1m 10s
gulp dist --extensions_from examples/article.amp.html	~2m	~1m 40s
gulp dist	~7m	~6m 30s
gulp dist --single_pass	~5m	~5m

Coming up:

• Enable a minified version of gulp with instant startup and lazy building (🏗✨ Enable lazy-build and watch modes for minified code #27471)

Addresses #26779 (comment)

[rsimha]

@erwinmombay @kristoferbaxter Extensively tested this locally, and it's now ready for a review. This will pave the way for minified lazy builds, and the pattern could potentially be adopted for targeted integration tests (--files) as well (they currently do an all-at-once transform).

[samouri]

Awesome PR. Thank you for speeding up builds! ⭐️

[jridgewell]

Can we prioritize #27161? We're adding more and more call-site configurations that will make it difficult to synchronize changes across environments.

[rsimha]

I believe @rcebulko is already working on this. (A fair bit has changed in recent days, so that PR will need a full rebase before proceeding. Shouldn't be bad though.)

[rcebulko]

Can/will do 👍