Improve error messages for naively used packages

[rbetts]

Examples like "https://jsr.io/@std/dotenv/doc/~/load#example_1..." don't work out of the box and give error messages with insufficient guidance:

 error: Relative import path "@std/dotenv/load" not prefixed with / or ./ or ../
    at file:///Users/rbetts/code/test-env/hello.ts:2:8

We should improve the CLI error message to tell people how to resolve the problem ("add the registry specifier or use deno add")?

Further, naively attempting deno add @std/dotenv/load produces a confusing error message that doesn't include guidance:

Created deno.json configuration file.
error: Failed to parse package required: @std/dotenv/load

Caused by:
    0: Invalid package requirement '@std/dotenv/load'. Invalid version requirement. Invalid specifier version requirement. Unexpected character '/'
         /load
         ~
    1: Invalid version requirement. Invalid specifier version requirement. Unexpected character '/'
         /load
         ~
    2: Invalid specifier version requirement. Unexpected character '/'
         /load
         ~
    3: Unexpected character '/'
         /load
         ~

[dsherret]

Moved out the second point to #25385