Support passing `--srcdir` to goimports

When using goimports via our gofmt formatter as [suggested in the docs](https://github.com/google/vim-codefmt/blob/1025977/doc/codefmt.txt#L42), the goimports path-based inference doesn't work because we don't tell goimports about the path. It does support a `--srcdir` argument we could pass.

We'd have to make it conditional and detect `basename(gofmt_executable) == 'goimports'` or something like that because gofmt doesn't support any `--srcdir` arg.

从 doc/codefmt.txt 中描述的 gofmt 格式化器用法开始，跟踪 gofmt_executable 的调用方式。检查 goimports 的路径推断行为和 --srcdir 选项；完成标准是仅向 goimports 传递该参数，而 gofmt 继续在不带该参数的情况下运行。

由索引模型根据 Issue 内容生成。