feat: Add a hint on error about 'Relative import path ... not prefixed with ...'

[bartlomieju]

Running a file like:

import "@std/dotenv/load";

Without a mapping in imports field of deno.json or dependencies of package.json
will now error out with a hint:

error: Relative import path "@std/dotenv/load" not prefixed with / or ./ or ../
  hint: Try running `deno add @std/dotenv/load`
    at [WILDCARD]bare_specifier_without_import/main.ts:1:8

Closes #24699