(feat): add --preserveModules flag

- this works the same way as Rollup's preserveModules option, leaving
  the module structure and not bundling any files together
  - format, env, and minify are used as directories instead of filename
    suffixes since the filename can't be changed without configuring
    output.entryFileNames (possibly a future improvement?)

src/createRollupConfig.ts

@@ -43,6 +43,18 @@ export async function createRollupConfig(
     .filter(Boolean)
     .join('.');
 
+  const outputDir = [
+    `${paths.appDist}/${opts.format}`,
+    opts.env,
+    shouldMinify ? 'min' : '',
+  ]
+    .filter(Boolean)
+    .join('/');
+
+  const output = opts.preserveModules
+    ? { dir: outputDir }
+    : { file: outputName };
+
   let tsconfigJSON;
   try {
     tsconfigJSON = await fs.readJSON(opts.tsconfig || paths.tsconfigJson);
@@ -79,10 +91,11 @@ export async function createRollupConfig(
       // Punchline....Don't use getters and setters
       propertyReadSideEffects: false,
     },
+    preserveModules: opts.preserveModules,
     // Establish Rollup output
     output: {
-      // Set filenames of the consumer's package
-      file: outputName,
+      // Set filenames or directories of the consumer's package
+      ...output,
       // Pass through the file format
       format: opts.format,
       // Do not let Rollup call Object.freeze() on namespace import objects

src/index.ts

@@ -285,6 +285,8 @@ prog
   .example('build --transpileOnly')
   .option('--extractErrors', 'Extract invariant errors to ./errors/codes.json.')
   .example('build --extractErrors')
+  .option('--preserveModules', 'Preserve module structure; do not bundle')
+  .example('watch --preserveModules')
   .action(async (dirtyOpts: WatchOpts) => {
     const opts = await normalizeOpts(dirtyOpts);
     const buildConfigs = await createBuildConfigs(opts);
@@ -393,6 +395,8 @@ prog
   .example(
     'build --extractErrors=https://reactjs.org/docs/error-decoder.html?invariant='
   )
+  .option('--preserveModules', 'Preserve module structure; do not bundle')
+  .example('build --preserveModules')
   .action(async (dirtyOpts: BuildOpts) => {
     const opts = await normalizeOpts(dirtyOpts);
     const buildConfigs = await createBuildConfigs(opts);

src/types.ts

@@ -1,4 +1,6 @@
 interface SharedOpts {
+  // Should preserve source structure? If true, files will not be bundled
+  preserveModules: false;
   // JS target
   target: 'node' | 'browser';
   // Path to tsconfig file

test/tests/tsdx-build.test.js

@@ -102,6 +102,32 @@ describe('tsdx build', () => {
     expect(code).toBe(0);
   });
 
+  it('should preserve modules after compilation', () => {
+    util.setupStageWithFixture(stageName, 'build-default');
+
+    const output = shell.exec(
+      'node ../dist/index.js build --format esm,cjs --preserveModules'
+    );
+
+    // ESM build
+    expect(shell.test('-f', 'dist/esm/index.js')).toBeTruthy();
+    expect(shell.test('-f', 'dist/esm/foo.js')).toBeTruthy();
+    expect(shell.test('-f', 'dist/esm/index.d.ts')).toBeTruthy();
+    expect(shell.test('-f', 'dist/esm/foo.d.ts')).toBeTruthy();
+    // CJS dev build
+    expect(shell.test('-f', 'dist/cjs/development/index.js')).toBeTruthy();
+    expect(shell.test('-f', 'dist/cjs/development/foo.js')).toBeTruthy();
+    expect(shell.test('-f', 'dist/cjs/development/index.d.ts')).toBeTruthy();
+    expect(shell.test('-f', 'dist/cjs/development/foo.d.ts')).toBeTruthy();
+    // CJS prod build
+    expect(shell.test('-f', 'dist/cjs/production/min/index.js')).toBeTruthy();
+    expect(shell.test('-f', 'dist/cjs/production/min/foo.js')).toBeTruthy();
+    expect(shell.test('-f', 'dist/cjs/production/min/index.d.ts')).toBeTruthy();
+    expect(shell.test('-f', 'dist/cjs/production/min/foo.d.ts')).toBeTruthy();
+
+    expect(output.code).toBe(0);
+  });
+
   afterEach(() => {
     util.teardownStage(stageName);
   });