x/tools/gopls: VS Code breadcrumbs broken with methods

[stamblerre]

See microsoft/vscode-go#3021.

[stamblerre]

After some quick investigation, it seems that this is caused by our use of the hierarchical symbol support to categorized methods below their receiver's type. The behavior of the VS Code extension is to show something like (*a).b at the top level, instead of a, which has a child symbol b(). I think that VS Code's breadcrumbs break because the child symbol's position is not contained within the range of the parent symbol. Based on that behavior, I think that we can fix this by treating methods as top-level symbols.