Fix errors in non-moduled dependencies

What's Observed
===============

Bingo returns errors when I open files in the $GOPATH/pkg/mod directory
(on Emacs with lsp-mode).

For example, textDocument/hover requests fail with the following error
message:

```
--> request #165: textDocument/hover:
{"textDocument":{"uri":"file:///home/yudai/gopath/pkg/mod/k8s.io/[email protected]/pkg/kubelet/pod/pod_manager.go"},"position":{"line":131,"character":0}}
<-- error #165: textDocument/hover: {"code":0,"message":"go [list -f
{{context.GOARCH}} {{context.Compiler}} -- unsafe]: exit status 1: go:
creating new go.mod: module k8s.io/kubernetes\ngo: copying requirements
from ../../../Godeps/Godeps.json\ngo: open
/home/yudai/gopath/pkg/mod/k8s.io/[email protected]/go.mod: permission
denied\n","data":null}
```

Root Cause
==========

The command `go [list -f {{context.GOARCH}} {{context.Compiler}} --
unsafe` is executed by `GetSizesGolist()` in golang.org/x/tools:
https://github.com/golang/tools/blob/79186431cf2917562ba2e632326924b5594ce114/go/internal/packagesdriver/sizes.go#L81

The `go list` command actually fails alone if you run it on the
directory of a non-moduled (no go.mod) dependency package.

```
$ echo $GO111MODULE
on
$ pwd
/home/yudai/gopath/pkg/mod/k8s.io/[email protected]
$ go list -f '{{context.GOARCH}} {{context.Compiler}}' --
unsafe
go: creating new go.mod: module k8s.io/kubernetes
go: copying requirements from Godeps/Godeps.json
go: open /home/yudai/gopath/pkg/mod/k8s.io/[email protected]/go.mod:
permission denied
```

It seems `go list ` tries to create `go.md` when  `GO111MODULE` is on
even if you are in `pkg/mod`. However since directories in `pkg/mod` are
read only, the command always fails.
I’m not sure if this behavior itself is correct (`go list` should not
try to convert packages to modules in `pkg/mod` even when MODULE is
on?), but anyway it won’t work in `pkg/mod` now.

This `GetSizesGoList()` is called from `packages.Load()` and it’s
originated from `*View.parse()` in `cache/view.go`. We currently set
`path` to `v.Config.Dir`, that makes the `go list` command run in `path`
and causes the error above.

Fix
===

We don’t need to set `v.Config.Dir`. The go command will be run in the
current working directory and that just works.

langserver/internal/cache/view.go

@@ -9,7 +9,6 @@ import (
 	"go/ast"
 	"go/parser"
 	"go/token"
-	"path/filepath"
 	"sync"
 
 	"github.com/saibing/bingo/langserver/internal/source"
@@ -83,7 +82,6 @@ func (v *View) parse(uri source.URI) error {
 		return err
 	}
 
-	v.Config.Dir = filepath.Dir(path)
 	v.Config.ParseFile = v.parseFile
 	pkgs, err := packages.Load(v.Config, fmt.Sprintf("file=%s", path))
 	if len(pkgs) == 0 {