x/tools/internal/imports: Grouping when using -local flag groups into more groups than expected

[willdot]

What version of Go are you using (go version)?

$ go version
go version go1.16.3 darwin/amd64

Does this issue reproduce with the latest release?

Yes

What operating system and processor architecture are you using (go env)?

go env Output

$ go env
GO111MODULE=""
GOARCH="amd64"
GOBIN=""
GOCACHE="/Users/will/Library/Caches/go-build"
GOENV="/Users/will/Library/Application Support/go/env"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOINSECURE=""
GOMODCACHE="/Users/will/go/pkg/mod"
GONOPROXY="github.com/bugsnag"
GONOSUMDB="github.com/bugsnag"
GOOS="darwin"
GOPATH="/Users/will/go"
GOPRIVATE="github.com/bugsnag"
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/usr/local/Cellar/go/1.16.6/libexec"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/usr/local/Cellar/go/1.16.6/libexec/pkg/tool/darwin_amd64"
GOVCS=""
GOVERSION="go1.16.6"
GCCGO="gccgo"
AR="ar"
CC="clang"
CXX="clang++"
CGO_ENABLED="1"
GOMOD="/dev/null"
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"

What did you do?

Started with imports, grouped like this:

 import (
	"fmt"
	
	"github.com/pkg/errors"
        "go.mongodb.org/mongo-driver/bson"
	
	"github.com/mymodule/somepkg"
) 

Added a new module import, which my IDE added to the middle group, so it looked like this

import (
	"fmt"
	
	"github.com/pkg/errors"
        "go.mongodb.org/mongo-driver/bson"
	"github.com/mymodule/anotherpkg"
	
	"github.com/mymodule/somepkg"
)

Then ran goimports -w -local github.com/mymodule main.go

What did you expect to see?

import (
	"fmt"
	
	"github.com/pkg/errors"
        "go.mongodb.org/mongo-driver/bson"
	
	"github.com/mymodule/anotherpkg"
	"github.com/mymodule/somepkg"
)

What did you see instead?

import (
	"fmt"
	
	"github.com/pkg/errors"
        "go.mongodb.org/mongo-driver/bson"
	
	"github.com/mymodule/anotherpkg"
	
	"github.com/mymodule/somepkg"
)

[cagedmantis]

/cc @heschi

[willdot]

I had a play with this and can't figure out why the code isn't working as expected, but I was able to create a unit test to show the issue on a fork of this repo.

https://github.com/willdot/tools/commit/5c95688d2e421215fa7ed8f510be4f4c65d82a01

@heschi @cagedmantis