x/tools/gopls: don't link command-line-arguments packages in hover

[findleyr]

This is a follow-up issue to fix some remaining bugs in hovering over command-line-arguments packages.

1. We should url-escape package paths.

2. We should not offer links to command-line-arguments packages, as they are not real.

          So, to summarize, there are two bugs here:
   
          1. gopls is not properly url-escaping package paths.
          3. ~gopls is not providing accurate hover information for builtins, when viewing the builtin file.~
   
          (1) should be fixed by a relatively straighforward application of url escaping. Assigning to our new team member @h9jiang as this is a good starter exercise for bug fixing and writing a test.
   
          On the other hand, (2) might require a more sweeping refactoring of how we handle the builtin file and unsafe packages, and is just about the polar opposite of a good starter bug: requires familiarity with nitty-gritty details of the codebase. I will do this part.
   

Originally posted by @findleyr in #68026 (comment)