Merge injected es6 imports

[JLHwung]

Q	A
Fixed Issues?
Patch: Bug Fix?
Major: Breaking Change?
Minor: New Feature?
Tests Added + Pass?	Yes
Documentation PR Link
Any Dependency Changes?
License	MIT

In this PR we try to multiple ES6 module imports based on the imported source. We track the last injected import in import-injector and persist that memory throughout the program's transpiling lifecycle.

The current implementation might break if there are external imports manipulation, i.e. users are modifying imports using tools other than helper-module-imports, because the injected import may not be the last import of the program and we will change the semantics when the imports have side effect. We could solve this by a new linear search to get the last import, but I think such cases are rare and we can postpone such fix to the future.

[babel-bot]

Build successful! You can test your changes in the REPL here: https://babeljs.io/repl/build/53795/

[JLHwung]

Some cases we can't optimize yet:

import * as react from "react";
import { jsx as _jsx } from "react/jsx-runtime";
import { createElement as _createElement } from "react";
import { jsxs as _jsxs } from "react/jsx-runtime";

Maybe we can introduce a new assumption like option (isolatedModule?) to allow addNamed() to merge imports more aggressively: not only merging to the last injected imports, but also any injected imports. As a jsx transformer we know that most jsx-runtime will satisfy this assumption and we can leverage that. General minifiers like terser will not combine such imports.

[nicolo-ribaudo]

This code:

import { A } from "a";
import { B } from "b";
import { C } from "a";

is always equivalent to

import { A, C } from "a";
import { B } from "b";

because dependencies gets evaluated in the order of the first import that references them.

What's the reason to not do the more aggressive optimization by default? Just because it's more performant to not track all the imports?

[JLHwung]

@nicolo-ribaudo You are right! And they are hoisted, now I am wondering why terser does not implement that optimization. If it is generally safe, I think it'd be implemented by a minifier rather than us.