x/tools/gopls: improve textDocument/documentLink

[stamblerre]

Some additional features we should add support for in textDocument/documentLink:

• Links missing http:// or https:// should be treated as links. For example, golang.org/issue/12345. (Completed via golang.org/cl/194661).
• Links to the Go issue tracker should be treated as links. For example, golang/go#12345.

Please add any others as ideas come up.

[eduvim]

I'm working on it

[gopherbot]

Change https://golang.org/cl/194537 mentions this issue: internal/links: Improve links parser, no protocol specification

[gopherbot]

Change https://golang.org/cl/194661 mentions this issue: internal/links: Improve links parser, no protocol specification