ui: Go explorer is visible when there is no go code

[hyangah]

golang.go-nightly v2022.4.1902

How to repro:

• Install golang.go-nightly.
• Create an empty directory
• Open code from the directory

I confirm that the extension is not activated - check Output > Log (Extension Host), or extensions view.
But the "Go" section is visible from explorer view.

@jamalc @suzmue Is it possible to make this viewlet visible conditionally? (i.e. setting context)

[gopherbot]

Change https://go.dev/cl/401614 mentions this issue: src/goExplorer.ts: show go explorer on extension activation