textDocument/completion: set CompletionItem.Documentation

[mbana]

WIP: Needs more work and input. I don't expect this PR to be approved, I guess I am just asking for input on how to improve the implementation

---

Changes:

• Remove LangHandler:getImportPackage and call goast.SearchImportPackage directly.
• Move LangHandler:findComments from langserver/hover.go to langserver/internal/source/hover.go as FindComments.
• Refactor FindComments and introduce new function PullComments.
• Move packageDoc from langserver/hover.go to langserver/internal/source/hover.go.
• Move joinCommentGroups from langserver/hover.go to langserver/internal/source/hover.go.

https://microsoft.github.io/language-server-protocol/specification#textDocument_completion

---

Screenshot:

---

See: https://github.com/saibing/bingo/issues/91

[saibing]

@mbana

thank your great work