Import overriding

[OdaDaisuke]

quikman overrides existing import dec like this.
https://github.com/OdaDaisuke/quikman/blob/master/cmd/quikman/main.go#L69

This logic has some problem.

CASE: No import dec

Example code is shown below.

package main

type XXXModel struct {
}

Output file goes like this.

package main

type XXXModel struct {
}

...auto generated code...

type XXXRepository struct {
  dbCtx *sql.DB
}

...

Generated code uses database/sql package, but the file has no import decl.