Chore: refactor normalizeOptions and the resolver pipeline

[fatfisz]

This is the last one of my refactors. Changes here:

• moving normalizeOptions to a separate file, which separates it from the visitor code. Makes the intent in index.js more clear IMO, because there is a lot less to look at now.
• making resolvers into a pipeline-ish thing: there was a pattern which was repeated three times and so it made sense to let JS go over the resolvers in the right order (a bit more clear now - one array with all of them) instead of repeating the code.
• moving types into state - a thing I experimented with in another plugin. It allows moving the visitor code out of the main export function, which I also did.
• adding more tests for yet undocumented scenarios.

Edit: added the point about types.
Edit: added the point about the tests.

[codecov]

Codecov Report

Merging #159 into beta will not change coverage.
The diff coverage is 100%.

Impacted Files	Coverage Δ
src/normalizeOptions.js	100% <100%> (ø)
src/getRealPath.js	100% <100%> (ø)	⬆️
src/index.js	100% <100%> (ø)	⬆️

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[codecov]

Codecov Report

Merging #159 into beta will not change coverage.
The diff coverage is 100%.

Impacted Files	Coverage Δ
src/transformers/call.js	100% <100%> (ø)	⬆️
src/getRealPath.js	100% <100%> (ø)	⬆️
src/index.js	100% <100%> (ø)	⬆️
src/utils.js	100% <100%> (ø)	⬆️
src/normalizeOptions.js	100% <100%> (ø)
src/transformers/import.js	100% <100%> (ø)	⬆️

---

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Δ = absolute <relative> (impact), ø = not affected, ? = missing data
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[tleunen]

Thanks @fatfisz. Looks good.