gopls/internal/mcp: add top level symbols from current package

When adding context against the current package based on the input
file URI, write both the exported and unexported symbols.

Providing context against the current file over the other files in
current package.

For golang/go#73580

Change-Id: Ica2166b8b005badcbf5b81e2714e5d326d29e7e0
Reviewed-on: https://go-review.googlesource.com/c/tools/+/677537
Auto-Submit: Hongxiang Jiang <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Alan Donovan <[email protected]>

Author: h9jiang