Skip installation of installed scoped packages.

Currently scoped packages will be reinstalled regardless whether they already exist in the node_modules tree.

This makes scoped packages skip reinstallation the same way normal, unscoped packages do.

e.g. 

#### `package.json`
```json
{
  "name": "scopedinstall",
  "version": "1.0.0",
  "dependencies": {
    "@fruit/apple": "file:apple",
    "cashew": "file:cashew"
  }
```

#### `./apple/package.json`

```json
{
  "name": "@fruit/apple",
  "version": "1.0.0"
}
```
#### `./cashew/package.json`

```json
{
  "name": "cashew",
  "version": "1.0.0"
}
```

## Before

Note `cashew` is only installed once, `@fruit/apple` is reinstalled every `npm install`.

```
> npm install --silent
cashew@1.0.0 node_modules/cashew

@fruit/apple@1.0.0 node_modules/@fruit/apple

> npm install --silent
@fruit/apple@1.0.0 node_modules/@fruit/apple

> npm install --silent
@fruit/apple@1.0.0 node_modules/@fruit/apple
```

## After
Note npm has ceased reinstalling `@fruit/apple` every `npm install`.

```
> npm install --silent
cashew@1.0.0 node_modules/cashew

@fruit/apple@1.0.0 node_modules/@fruit/apple
> npm install --silent

> npm install --silent

```
Is it necessary to add a whole extra asyncMap wrapped around the existing one? Couldn't the resolved, scoped packages just be concatenated to the existing list of packages (or couldn't all of the packages get resolved at this point)?