build: merge webpack configurations (pro and dev) to define only once in angular.json

[Alexis Georges]

I'm submitting a...

[ ] Regression (a behavior that used to work and stopped working in a new release)
[ ] Bug report  
[X] Feature request
[ ] Documentation issue or request
[ ] Support request => Please do not submit support request here, instead see https://github.com/NationalBankBelgium/stark/blob/master/CONTRIBUTING.md#got-a-question-or-problem

Current behavior

Need to define webpack configurations twice in "angular.json" file:

"$schema": "./node_modules/@angular/cli/lib/config/schema.json",
  "version": 1,
  "newProjectRoot": "projects",
  "projects": {
    "showcase": {
      "architect": {
        "build": {
          "builder": "@angular-builders/custom-webpack:browser",
          "options": {
            "customWebpackConfig": {
              "path": "./node_modules/@nationalbankbelgium/stark-build/config/webpack-partial.dev.js",
              // ...
            }
          "configurations": {
              "customWebpackConfig": {
                "path": "./node_modules/@nationalbankbelgium/stark-build/config/webpack-partial.prod.js",
                // ...
              }
          }
        }
      }
    }
  }
}

Expected behavior

As metadata is now improved after upgrade to ng8 (see: #2550), we can now have only one configuration for webpack.
That would allow us to define only once the webpack configuration in "angular.json":

"$schema": "./node_modules/@angular/cli/lib/config/schema.json",
  "version": 1,
  "newProjectRoot": "projects",
  "projects": {
    "showcase": {
      "architect": {
        "build": {
          "builder": "@angular-builders/custom-webpack:browser",
          "options": {
            "customWebpackConfig": {
              "path": "./node_modules/@nationalbankbelgium/stark-build/config/webpack.config.js",
              // ...
            }
          }
        }
      }
    }
  }
}

What is the motivation / use case for changing the behavior?

Simplify and improve