fix: mock package json instead of fs

src/sfdxProject.ts

@@ -236,7 +236,7 @@ export class SfdxProjectJson extends ConfigFile<ConfigFile.Options> {
       // Always end in a path sep for standardization on folder paths
       const fullPath = `${dirname(this.getPath())}${sep}${name}${sep}`;
 
-      if (!fs.existsSync(fullPath)) {
+      if (!this.doesPackageExist(fullPath)) {
         throw new SfdxError(
           'InvalidPackageDirectory',
           this.messages.getMessage('InvalidPackageDirectory', [packageDir.path])
@@ -304,6 +304,10 @@ export class SfdxProjectJson extends ConfigFile<ConfigFile.Options> {
     return this.getContents().packageDirectories && this.getContents().packageDirectories.length > 1;
   }
 
+  private doesPackageExist(packagePath: string) {
+    return fs.existsSync(packagePath);
+  }
+
   private validateKeys(): void {
     // Verify that the configObject does not have upper case keys; throw if it does.  Must be heads down camel case.
     const upperCaseKey = sfdc.findUpperCaseKeys(this.toObject(), SfdxProjectJson.BLOCKLIST);

src/testSetup.ts

@@ -33,9 +33,8 @@ import { Crypto } from './crypto';
 import { Logger } from './logger';
 import { Messages } from './messages';
 import { SfdxError } from './sfdxError';
-import { SfdxProject } from './sfdxProject';
+import { SfdxProject, SfdxProjectJson } from './sfdxProject';
 import { CometClient, CometSubscription, StreamingExtension } from './status/streamingClient';
-import { fs } from './util/fs';
 
 /**
  * Different parts of the system that are mocked out. They can be restored for
@@ -345,7 +344,7 @@ export const stubContext = (testContext: TestContext) => {
     testContext.rootPathRetrieverSync(isGlobal, testContext.id)
   );
 
-  testContext.SANDBOXES.DEFAULT.stub(fs, 'existsSync').callsFake(() => true);
+  stubMethod(testContext.SANDBOXES.PROJECT, SfdxProjectJson.prototype, 'doesPackageExist').callsFake(() => true);
 
   const initStubForRead = (configFile: ConfigFile<ConfigFile.Options>): ConfigStub => {
     const stub: ConfigStub = testContext.configStubs[configFile.constructor.name] || {};