Improve prepack script (#1747)

* improve prepack script

* remove .npmignore

* make prepack use pkg.files

* fix linter errors

scripts/prepack.js

@@ -0,0 +1,42 @@
+#!/usr/bin/env node
+
+// This script removes the build artifacts of ignored contracts.
+
+const fs = require('fs');
+const path = require('path');
+const cp = require('child_process');
+const match = require('micromatch');
+
+function exec (cmd, ...args) {
+  cp.execFileSync(cmd, args, { stdio: 'inherit' });
+}
+
+function readJSON (path) {
+  return JSON.parse(fs.readFileSync(path));
+}
+
+exec('npm', 'run', 'compile');
+
+const pkgFiles = readJSON('package.json').files;
+
+// Get only negated patterns.
+const ignorePatterns = pkgFiles
+  .filter(pat => pat.startsWith('!'))
+// Add **/* to ignore all files contained in the directories.
+  .flatMap(pat => [pat, path.join(pat, '**/*')])
+// Remove the negation part. Makes micromatch usage more intuitive.
+  .map(pat => pat.slice(1));
+
+const artifactsDir = 'build/contracts';
+
+for (const artifact of fs.readdirSync(artifactsDir)) {
+  const fullArtifactPath = path.join(artifactsDir, artifact);
+  const { sourcePath: fullSourcePath } = readJSON(fullArtifactPath);
+  const sourcePath = path.relative('.', fullSourcePath);
+
+  const ignore = match.any(sourcePath, ignorePatterns);
+
+  if (ignore) {
+    fs.unlinkSync(fullArtifactPath);
+  }
+}