Generate package.json with correct path to the main entry file

[jozefizso]

Fixes #246 Wrong main entry in the generated package.json

[JoshuaKGoldberg]

Just posting for visibility: I want to review this and it looks like a quick 'yes'. But...

1. The changes can't be easily tested pending updating the yeoman-generator dependency: instantiatedGenerator.run is not a function #258
2. That issue is blocked on require(ESM): Switch from CJS (CommonJS) to ESM (ECMAScript Modules) #265
   • ...which, now that Node.js LTS supports it, might be less difficult than a full feat!: switch from CJS to ESM and bump Yeoman packages #269 - TBD