vsce package defaults to using wrong package manager

[ntilwalli]

• VSCE Version: 1.81.0
• OS Version: MacOS 10.15.6

Steps to Reproduce:

1. At command-line run yo code and create the default hello world extension (choose npm as package manager, not yarn)
   add a publisher property to package.json
2. Install any package in "dependencies" (i.e. npm install --save html-to-hyperscript)
3. Run vsce package
4. Notice how it uses yarn when I have told it to use npm. It says...

Executing prepublish script 'yarn run vscode:prepublish'...
yarn run v1.22.5

In using the wrong package manager it also bundles the package improperly by skipping including dependencies from node_modules

The workaround is to always explicitly either use the --yarn or --no-yarn option when running vsce package. When the package manager is included explicitly, the node_modules dependencies are properly included.

[ntilwalli]

Closing since seems related to #494