fix(bin): extension detection (#724)

[lakatostamas]

What kind of change does this PR introduce?
bugfix

Did you add tests for your changes?
No, but I created test repositories:
https://github.com/lakatostamas/webpack-babel
https://github.com/lakatostamas/webpack-ts

Summary
Issue: #724
This commit introduced a bug in the default config file discovery: b61220e#diff-daabbe74e88640829a3ad9e9899efd8cR79
Before this commit, the extension of webpack.config.ts was .ts in the default configFile discovery, but after this commit, it became just ts (without .). webpack.config.babel.js extension is js now, but it should be .babel.js. With this pull request, I introduce a fix, which gets the name of the default config file and removes the webpack.config or webpackfile from it.
I don't know how to add tests when the extension of webpack config is not js. I haven't found any example, but if you tell me how to do it, I would happily add some tests to cover these cases.

Does this PR introduce a breaking change?
No

[zzh8829]

This should fix the problem of .babel.js discussed in webpack/webpack#5960
Looks like the error was actually introduced in 14ec0a5#diff-daabbe74e88640829a3ad9e9899efd8cR90

[ematipico]

We should add a bin test for that, if possible

[webpack-bot]

@lakatostamas Thanks for your update.

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

@ematipico Please review the new changes.

[ematipico]

Thanks!

[lakatostamas]

@ematipico Thanks for the review! The webpack-babel-config test works because in the project root we have a .babelrc and @babel/preset-env is installed. I added a .babelrc to webpack-babel-config root to be more explicit, but this test will fail if the preset-env dependency won't be installed.
If we want to add webpack.config.ts related tests, then we have to add ts-node as a devDependency, but I think it's not worth for one test.

[ematipico]

I personally prefer install an additional devDependency and make sure that we don't break those kind of scenarios. After all this is a package used by lot of people :) what do you think?

[lakatostamas]

Okay, that makes sense. I added webpack.config.ts test

[webpack-bot]

@lakatostamas The most important CI builds failed. This way your PR can't be merged.

Please take a look at the CI results from travis (failure) and fix these issues.