fix(generators): fix and refactor entry util, add tests

[knagaitsev]

What kind of change does this PR introduce?
fix, refactor

Did you add tests for your changes?
Yes

If relevant, did you update the documentation?
N/A

Summary

The generators entry util was overly complicated and repetitive. One strange piece of logic in it was this:

webpack-cli/packages/generators/src/utils/entry.ts

Line 38 in e232d0e

n[val].charAt(0) !== "(" &&

. I think this was intended for if the user wanted to provide an entry that would be executed, for example path.join(__dirname, 'index.js'). However, allowing this functionality makes no sense given how the init generator will try to write a file using this path (not what this line executes to, but the actual string "path.join(__dirname, 'index.js')", since this would be what the user provided on command line - an unlikely scenario).

I did the following:

• removed checks to see if the user provided a path that is meant to be executed as mentioned above
• made the return value an empty string for single entry if the user provides that (not doing this was a bug)
• fix the users provided path depending on how they formatted it. If they provide src/index then it will be adjusted to ./src/index.js. But if the user provides ./src/index.js, nothing is changed

Does this PR introduce a breaking change?
Yes, as mentioned above

Other information

[alexander-akait]

Thanks

[webpack-bot]

@jamesgeorge007 Thanks for your update.

I labeled the Pull Request so reviewers will review it again.

@evilebottnawi Please review the new changes.