Merging imports with different names breaks programs

[jjant]

Bug description

Running elm-format on

import Html
import Html as Html_

Results in

import Html as Html_

Which breaks all the Html.x names. This is even more notable in cases like

import Html as Html1
import Html as Html2
import Html as Html3
import Html as Html_

Where it just keeps the last one.

Proposed solution

Either don't merge aliased imports like these at all, or look at the bindings to check unused aliases and then do it (this is probably needlessly complicated).

Caveat

Note that export lists are always safe to merge, i.e, you can turn this:

import Html as Html1 exposing (a, div)
import Html as Html2 exposing (input)
import Html as Html3
import Html as Html_

Into this (or add a plain import Html exposing (a, div, input) at the top for consistency):

import Html as Html1 exposing (a, div, input)
import Html as Html2
import Html as Html3
import Html as Html_