Emit declaration files into the correct location based on WebPack's context

[bryanforbes]

Given the following project structure:

project/
    src/
        thing1.ts
        thing2.ts

and the following WebPack configuration:

module.exports = {
    context: 'src',
    entry: {
        thing1: 'thing1.ts',
        thing2: 'thing2.ts'
    },
    output: {
        path: path.join(__dirname, 'dist'),
        filename: '[name].js',
        libraryTarget: 'umd'
    },
    resolve: {
        extensions: [ '', '.ts', '.js' ],
        root: 'src'
    },
    module: {
        loaders: [
            {
                test: /\.ts$/,
                loader: 'awesome-typescript-loader',
                query: {
                    declaration: true
                }
            }
        ]
    }
};

The following directory structure is created:

project/
    dist/
        src/
            thing1.d.ts
            thing2.d.ts
        thing1.js
        thing2.js

This pull request updates the declaration file destination calculation to use WebPack's loaderUtils to determine where to emit the declaration file. This pull request does not cover "elided" declaration files which will still output with the incorrect path.

[s-panferov]

@bryanforbes thanks for the PR! I need to make some research to find the best way to store declaration files. I'll try not to prolong this.