Don't hard code main.js, use main file from package.json

[jancurn]

When you run apify init in a normal Node.js project with other main file than 'main.js', it succeeds, but then when you run apify run you get this error:

Error: The "main.js" file not found in the current directory. Call "apify init" to create it.

We shouldn't hard-code "main.js" anywhere, user can use any file. Just read it from package.json