Prevent double application of the transform

[fatfisz]

There were problems described e.g. in #167 wrt. the possible double application stemming from a not-exactly-good configuration. I think we should warn when this may happen and consider this an anti-pattern.

Example of such configuration:

alias: {
  'react-native-vector-icons': '@expo/vector-icons',
  '@expo/vector-icons/lib/create-icon-set': 'react-native-vector-icons/lib/create-icon-set',
  '@expo/vector-icons/lib/icon-button': 'react-native-vector-icons/lib/icon-button'
}

If there's a path react-native-vector-icons/lib/create-icon-set then first application of the plugin will transform it to @expo/vector-icons/lib/create-icon-set, and another application will result in react-native-vector-icons/lib/create-icon-set. This is actually causing trouble for users in some cases.

---

I feel the message could be greatly improved. Any suggestions?

Also this may deserve a section in the README describing the problem, which we could refer to in the message.

[codecov]

Codecov Report

Merging #176 into master will not change coverage.
The diff coverage is 100%.

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

[tleunen]

Maybe I need to read the thread again but I'm still having hard times understanding why someone would compile the code twice. Therefore I'm keeping this open for now, I'll read the thread again.

[fatfisz]

In some cases because of a misconfiguration the plugin may run twice (or possibly more) during one compilation. I'll try to come up with a minimal PoC later this day.

[fatfisz]

@tleunen I've updated this branch instead of making a new PR to resuse the test. I've used pathResolved instead of seen to reduce the possibility of a conflict with other plugins.

Only new tests added, the previous ones did not break (even with the CJS transform).

[tleunen]

Perfect, lets get this in. Thank you @fatfisz !