x/tools/gopls: handle DocumentSymbol request using syntax info

[hyangah]

Currently gopls uses type information in populating DocumentSymbol responses.
As a result, it is available only if the file is in the current workspace or build set.
For example, if the current file is excluded by build tag or GOOS/GOARCh constraints
DocumentSymbol is completely empty.

Editors and plugins like VS Code and VS Code Go heavily rely on DocumentSymbol
(e.g. Test function discovery, Breadcrumb, ...) - mostly for go file parsing info.
Completely omitting DocumentSymbol limits the utility.

From a quick investigation, the type info provides very little additional info for
DocumentSymbol, so consider changing the implementation to use only syntax info.

[gopherbot]

Change https://go.dev/cl/405254 mentions this issue: internal/lsp/source: derive document symbols from syntax alone

[findleyr]

While there's a CL pending for this, there are enough outstanding questions that let's wait until v0.9.1, since this isn't critical.