internal/lsp: change to helper.go to output a formatted file
internal/lsp: change to helper.go to output a formatted file

Use format.Source before output.

Change-Id: I8504fc80defa77aeaab748b9dc65d6382da500a3
Reviewed-on: https://go-review.googlesource.com/c/tools/+/217177
Reviewed-by: Rebecca Stambler <d7cf11e5f299d88ea8348366a9fc4f7c335c3afa@golang.org>
Run-TryBot: Rebecca Stambler <d7cf11e5f299d88ea8348366a9fc4f7c335c3afa@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

golang/tools,golang/tools,golang/tools,golang/tools,golang/tools