x/tools/gopls: support renaming an embedded field

[myitcv]

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

$ go version
go version devel +f4b918384d Tue Mar 23 05:12:39 2021 +0000 linux/amd64
$ go list -m golang.org/x/tools
golang.org/x/tools v0.1.1-0.20210319205745-d7a25adab0d9
$ go list -m golang.org/x/tools/gopls
golang.org/x/tools/gopls v0.0.0-20210319205745-d7a25adab0d9

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="on"
GOARCH="amd64"
GOBIN=""
GOCACHE="/home/myitcv/.cache/go-build"
GOENV="/home/myitcv/.config/go/env"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOINSECURE=""
GOMODCACHE="/home/myitcv/gostuff/pkg/mod"
GONOPROXY=""
GONOSUMDB=""
GOOS="linux"
GOPATH="/home/myitcv/gostuff"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/home/myitcv/gos"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/home/myitcv/gos/pkg/tool/linux_amd64"
GOVCS=""
GOVERSION="devel +f4b918384d Tue Mar 23 05:12:39 2021 +0000"
GCCGO="gccgo"
AR="ar"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD="/home/myitcv/gostuff/src/github.com/myitcv/govim/go.mod"
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -m64 -pthread -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build1525600331=/tmp/go-build -gno-record-gcc-switches"

What did you do?

Follow up from #43616 (comment) (cc @mdempsky)

Relatively frequently I find myself in the following situation:

type T int

type S struct {
    T
}

With the cursor on the embedded T in S.

I then initiate a rename, but get:

gopls.Rename failed: can't rename embedded fields: rename the type directly or name the field

The fix in this instance is to jump to the definition of T, rename, then jump back. But obviously gopls can do that 😄

To my mind this case is distinct from the case of selector expressions mentioned in #43616 (comment): such a situation is more ambiguous because we can't be sure the user knows the field in question is an embedded field.

But in this case our cursor is on the declaration of the embedded field, so there can be no ambiguity.

cc @findleyr

FYI @leitzler