fix(gatsby-plugin-postcss): Always ensure postcss.config.js is loaded

[mdreizin]

Description

If you specify cssLoaderOptions then postcss-loader wound not picked up postcss.config.js config.

The postcssOptions options would contain cssLoaderOptions property ({cssLoaderOptions: {...}}) and postcss-loader gets it as is and does not use some built-in defaults.

This PR aims to fix that and also add some code-style adjustments to align with existing codebase.

Related Issues

#10861

[sidharthachatterjee]

Thank you for fixing this! Could you please add a test that asserts the correct behaviour here so that we don't see regressions in the future?

[mdreizin]

Thank you for fixing this! Could you please add a test that asserts the correct behaviour here so that we don't see regressions in the future?

Yes, I could. I will do my best to add missing tests this week.

[pieh]

Hey @mdreizin! Do you think you will have time to take a look at those tests? If not - that's not a problem - I can add test case for cssLoaderOptions. If you want to do this, but have any questions - don't hesitate to ask!

[mdreizin]

@pieh No, I don’t. I will be able to write them only at the weekend. It would be nice if you could add missing tests.

[sidharthachatterjee]

Thanks for the tests @pieh 💜

[pieh]

Diff for snapshot file is messy, mostly because we didn't reset mocked calls before.

Here's somewhat easier to see snapshot diff between master and this PR - https://gist.github.com/pieh/6ee5420e34bd61ca36d4392cbd64171c