fix: resolve pathes relative to ng-package.json

Previously, paths were resolved relative to the process's current working directory.
Now, paths are resolved relative to the `ng-package.json` file, making it more visible and intuitive to users.
This is in line with `.angular-cli.json`, `tsconfig.json` and friends who resolve pathes relative to the json files.

lib/interfaces.ts

@@ -1,11 +1,11 @@
 /** CLI arguments passed to `ng-packagr` and `ngPackage()`. */
 export interface NgPackagrCliArguments {
-  /** Path to the '.ng-packagr.json' file */
+  /** Path to the 'ng-package.json' file */
   project: string
 }
 
-/** Config object from '.ng-packagr.json' */
-export interface NgPackagrConfig {
+/** Config object from 'ng-package.json' */
+export interface NgPackageConfig {
   src: string,
   dest: string,
   workingDirectory: string,

lib/ng-packagr.ts

@@ -2,7 +2,7 @@
 import { processAssets } from './steps/assets';
 import { copyFiles } from './steps/copy';
 import { ngc } from './steps/ngc';
-import { createPackage, readJson, preparePackage } from './steps/package';
+import { createPackage, readJson, preparePackage, readPackage } from './steps/package';
 import { rimraf } from './steps/rimraf';
 import { rollup } from './steps/rollup';
 import { remapSourcemap } from './steps/sorcery';
@@ -13,22 +13,26 @@ import { downlevelWithTsc } from './steps/tsc';
 import { error, warn, info, success, debug } from './util/log';
 
 // Interfaces
-import { NgPackagrConfig, NgPackagrCliArguments } from './interfaces';
+import { NgPackageConfig, NgPackagrCliArguments } from './interfaces';
 
 // There are no type definitions available for these imports.
 const fs = require('mz/fs');
+const path = require('path');
 
 
 export const ngPackage = (opts: NgPackagrCliArguments): Promise<any> => {
   info(`Building Angular library from ${opts.project}`);
+  if (!path.isAbsolute(opts.project)) {
+    opts.project = path.resolve(process.cwd(), opts.project);
+  }
 
   /** Project configuration */
-  let project: NgPackagrConfig;
+  let project: NgPackageConfig;
   let tsConfig: any;
   let flatModuleFile: string;
   let sourcePkg: any;
 
-  return readJson(opts.project)
+  return readPackage(opts.project)
     .then((p) => {
       project = p;
 
@@ -46,7 +50,7 @@ export const ngPackage = (opts: NgPackagrCliArguments): Promise<any> => {
         return Promise.resolve(pkg);
       })
     )
-    .then(() => readJson(`${project.src}/${project.ngc.tsconfig}`)
+    .then(() => readJson(`${project.ngc.tsconfig}`)
       .then((cfg) => {
         tsConfig = cfg;
 
@@ -56,7 +60,7 @@ export const ngPackage = (opts: NgPackagrCliArguments): Promise<any> => {
     // 2. ASSETS
     .then(() => processAssets(project.src, `${project.workingDirectory}`))
     // 3. NGC
-    .then(() => ngc(`${project.src}/${project.ngc.tsconfig}`, `${project.workingDirectory}`))
+    .then(() => ngc(`${project.ngc.tsconfig}`, `${project.workingDirectory}`))
     .then(() => remapSourcemap(flatModuleFile))
     // 4. FESM15: ROLLUP
     .then(() => rollup({

lib/steps/package.ts

@@ -1,3 +1,4 @@
+import { NgPackageConfig } from '../interfaces';
 const fs = require('fs');
 const path = require('path');
 
@@ -27,9 +28,30 @@ export const readJson = (file: string): Promise<any> => {
 }
 
 
+/**
+ * Reads an Angular package definition file from 'ng-package.json'
+ *
+ * @param package `ng-package.json` definition file
+ */
+export const readPackage = (ngPkg: string): Promise<NgPackageConfig> => {
+  const base = path.dirname(ngPkg);
+
+  return readJson(ngPkg)
+    .then((p: NgPackageConfig) => {
+      // resolve pathes relative to `ng-package.json` file
+      p.src = path.resolve(base, p.src);
+      p.dest = path.resolve(base, p.dest);
+      p.workingDirectory = path.resolve(base, p.workingDirectory);
+      p.ngc.tsconfig = path.resolve(base, p.ngc.tsconfig);
+      p.rollup.config = path.resolve(base, p.rollup.config);
+
+      return Promise.resolve(p);
+    });
+}
+
 
 /**
- * Prepares a package...
+ * Prepares an Angular package from reading 'package.json'
  *
  * @param src Source directory containg `package.json` file
  */