Useless repeated imports when using external modules

## Setup
```sh
npx esbuild --version
# 0.7.17

echo 'export const x = 1' > a.js
echo 'import {x} from "./a.js"; console.log(x);' > b.js
echo 'import {x} from "./a.js"; console.log(x);' > c.js
echo 'import "./b.js"; import "./c.js";' > d.js
```

## Actual behavior
```
$ npx esbuild --bundle --format=esm --external:"`pwd`/a.js" d.js
// b.js
import {x} from "./a.js";
console.log(x);

// c.js
import {x as x2} from "./a.js";
console.log(x2);
```
And
```
$ npx esbuild --bundle --minify --format=esm --external:"`pwd`/a.js" d.js
import{x as o}from"./a.js";console.log(o);import{x as m}from"./a.js";console.log(m);
```
**NB:** `x` is imported twice, once as `x` and once as `x2`

## Expected behavior
```
$ npx esbuild --bundle --format=esm --external:"`pwd`/a.js" d.js
// b.js
import {x} from "./a.js";
console.log(x);

// c.js
console.log(x);
```
And
```
$ npx esbuild --bundle --minify --format=esm --external:"`pwd`/a.js" d.js
import{x as o}from"./a.js";console.log(o);console.log(o);
```
**NB:** `x` is imported only once

Re-run the provided esbuild 0.7.17 reproduction with external a.js and inspect the bundling path for repeated imports from the same external module. Compare the non-minified and minified output with the expected output, where the external export is imported once and both uses share it.

Written by the indexing model from the issue text.