AST duplication compare only by paths, not by alias. Fix it

[TheZeroSlave]

HI. AST removing duplicates works not correct for local aliasing. If you have several imports ta package with different local names - it will bring some pain like "unknown package" because ast remove all packages with another local naming. From the library source: go/ast/filter.go:458

        seen := make(map[string]bool)
        for _, filename := range filenames {
            f := pkg.Files[filename]
            for _, imp := range f.Imports {
                if path := imp.Path.Value; !seen[path] {
                    // TODO: consider handling cases where:
                    // - 2 imports exist with the same import path but
                    //   have different local names (one should probably
                    //   keep both of them)
                    // - 2 imports exist but only one has a comment
                    // - 2 imports exist and they both have (possibly
                    //   different) comments
                    imports = append(imports, imp)
                    seen[path] = true
                }
            }
        }

So this PR tries to fix this by handling duplicates by skipping them.

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[TheZeroSlave]

@googlebot I signed it!

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[codyoss]

Closing due to lack of response.