Use qualified module name from diagnostics in suggestNewImport

[berberman]

GHC helps us to split the qualified module name from the identifier. Closes haskell/haskell-language-server#629

[pepeiborra]

Haven't looked at the code yet, but haskell/haskell-language-server#629 is about operators and the title of this PR "Use qualified module name in suggestNewImport" doesn't seem related.

[berberman]

Ah, the issue was caused by we mistakenly stripped the qualified import prefix of identifiers. For example, if I write T.null , where the T is not in scope, diagnostic will report:

Not in scope: ‘T.null’
No module named ‘T’ is imported.

The previous logic only took the first line, then tried to use splitOn "." to strip T., obtaining the identifier null, which we use to lookup the export map. Everything goes well.

But operators like .|. were split unexpectedly, result in |. as the identifer. In this PR, I try to extract the qualified module name from the second line of the diagnostic message, so we no longer to use splitOn and intercalate manually, removing the qualified module name by T.stripPrefix instead.

[berberman]

Looks like the CI broke for some unknown reasons, but all checks passed on my fork: https://github.com/berberman/ghcide/tree/patch-qual
Ready to be reviewed and merged :-)

[pepeiborra]

Thank you, this is an awesome bug fix.

I don't know what the problem is with CI, could you perhaps rebase on top of master?

[pepeiborra]

/azp run

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