equally named private methods in different files break on merge

## Bug Report

If you have two files, each with a different class, but the classes share a method with the same name, the merged output file breaks because of name overlap.

- [x] I would like to work on a fix!

**Input Code**

```js
// A.js
class A {
constructor() {}
public() { return this.#private(); }
#private() { console.log('A private'); }
}

// B.js
class B {
constructor() {}
public() { return this.#private(); }
#private() { console.log('B private'); }
}

// main.js
// Expected: "A private"
// Actual: "B private"
(new A).public();
```
Also see [this repo](https://github.com/Cxarli/babel-private-method-duplicate-issue)

**Run Command**
`npx babel A.js B.js main.js -o out.js`

**Output Code**

(slightly trimmed to make it clearer what the issue is, not actual output)

```js
var _private = new WeakSet();
class A {
constructor() { _private.add(this); }
public() { return _classPrivateMethodGet(this, _private, _private2).call(this); }
}
var _private2 = function _private2() { console.log('A private'); };

// this ID is reused
var _private = new WeakSet();
class B {
constructor() { _private.add(this); }
public() { return _classPrivateMethodGet(this, _private, _private2).call(this); }
}
// this ID is reused
var _private2 = function _private2() { console.log('B private'); };
```

**Expected behavior**
The second pair should get unique names, so they don't clash, which would make the result `"A private"`

**Babel Configuration**
- Filename: `babel.config.js`
```js
module.exports = {
plugins: [
[require("@babel/plugin-proposal-class-properties")],
[require("@babel/plugin-proposal-private-methods")],
],
};
```

**Environment**
```
Binaries:
Node: 14.7.0 - /usr/bin/node
Yarn: 1.22.4 - /usr/bin/yarn
npm: 6.14.7 - /usr/bin/npm
npmPackages:
@babel/cli: ^7.10.5 => 7.10.5
@babel/core: ^7.11.1 => 7.11.1
@babel/plugin-proposal-class-properties: ^7.10.4 => 7.10.4
@babel/plugin-proposal-private-methods: ^7.10.4 => 7.10.4
```

**Possible Solution**
Technically you can first concat the files before putting it through babel with `npx babel <(cat A.js B.js main.js) -o out.js` which means they are in the same scope and get unique functions and everything works. However, then you lose all source information (file, line) in case of errors.