Fix Go imports

[egiptipavel]

Description

• Fixed bugs with imports described in issues
• Moved representation of GoUtModel's in Go code to GoUtModelToCodeConverter
• Created GoImportsResolver class for working with imports.

Fixes #1776
Fixes #1777

How to test

Manual tests

Tested on

package simple

import (
	"errors"
	_ "errors"
	e "errors"
	"github.com/pmezard/go-difflib/difflib"
	d "github.com/pmezard/go-difflib/difflib"
	"math"
	. "math"
)

func Test(match difflib.Match) d.Match {
	e.New("")
	errors.New("")
	n := math.NaN()
	IsNaN(n)
	return match
}

Self-check list

• I've set the proper labels for my PR (at least, for category and component).
• PR title and description are clear and intelligible.
• I've added enough comments to my code, particularly in hard-to-understand areas.
• The functionality I've repaired, changed or added is covered with automated tests.
• Manual tests have been provided optionally.
• The documentation for the functionality I've been working on is up-to-date.