feat: Add types-import missing for go code generator.

[jianjungki]

modified AbstractGoCodegen class for go

• update types-mapping logic for GoCodeGen, remove duplicate imports, and avoiding missing import in code.
• because const type can't not use self-defined type, so overwrite fromModel func for that.

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target language: Go
@antihax (2017/11) @grokify (2018/07) @kemokemo (2018/09) @jirikuncar (2021/01) @ph4r5h4d (2021/04)

[wing328]

@jianjungki thanks for the PR. Do you have a minimal spec to reproduce the issue? I think we need to some tests for this change.

[jianjungki]

@jianjungki thanks for the PR. Do you have a minimal spec to reproduce the issue? I think we need to some tests for this change.

okay, I will add the spec for these problem.