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

```
yeah it looks ugly