adjust imports on save

[bhcleek]

Introduce a new option, g:go_imports_autosave, to allow imports to be
adjusted even when g:go_fmt_command is not set to goimports.

Closes #2795

[bhcleek]

@orlangure I think the recent changes on master have taken care of the issues that you saw on #2797. Feel free to give this a run through if you want.

[orlangure]

Found two issues (and one probably related to gopls):

1. When creating a new file and using both fmt and imports on save (enabled by default), the first save causes error since the file does not exist, and the next save works.
2. When using the same options, every save leads to Press ENTER or type command to continue.

The last issue is inconsistency, I think it should be reported to gopls, but I'll post here anyway:

package main

import (
	"time"
	"github.com/orlangure/gnomock"
)

func meh() {
	_ = 0
	_ = gnomock.DefaultPort
	_ = time.Now()
}

Saving this file changes the order of imports to

import (
	"github.com/orlangure/gnomock"
	"time"
)

And saving this file changes it to

import (
	"time"

	"github.com/orlangure/gnomock"
)

[bhcleek]

The first item, the error on the first save, is an issue on master that I'll address in a separate PR.

I can't duplicate the second item.

I can duplicate the third item; based on the debug output (let g:go_debug=["lsp"]), it looks like a gopls issue.