x/tools/gopls: support setting imports LocalPrefix

[Zariel]

Please answer these questions before submitting your issue. Thanks!

What did you do?

With vscode it is possible to set formatTool flags which for goimports allows configuring "-local" which sets LocalPrefix in the imports.ProcessEnv struct passed in to the Options when fixing imports.

It is not possible to configure the imports behaviour when using gopls

What did you expect to see?

Local imports sorted outside of standard library ones

What did you see instead?

Local imports mixed with standard library

System details

go version go1.12.8 darwin/amd64
GOARCH="amd64"
GOBIN=""
GOCACHE="/Users/chris/Library/Caches/go-build"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOOS="darwin"
GOPATH="/Users/chris/dev/go"
GOPROXY=""
GORACE=""
GOROOT="/usr/local/Cellar/go/1.12.8/libexec"
GOTMPDIR=""
GOTOOLDIR="/usr/local/Cellar/go/1.12.8/libexec/pkg/tool/darwin_amd64"
GCCGO="gccgo"
CC="clang"
CXX="clang++"
CGO_ENABLED="1"
GOMOD=""
GOROOT/bin/go version: go version go1.12.8 darwin/amd64
GOROOT/bin/go tool compile -V: compile version go1.12.8
uname -v: Darwin Kernel Version 18.7.0: Thu Jun 20 18:42:21 PDT 2019; root:xnu-4903.270.47~4/RELEASE_X86_64
ProductName:	Mac OS X
ProductVersion:	10.14.6
BuildVersion:	18G87
lldb --version: lldb-1001.0.13.3
  Swift-5.0

[suzmue]

Duplicate of #32049

Feel free to reopen if you think this is a separate issue!