Fix #11107 - don't prefetch preloaded modules

[Sheraff]

This PR proposes a fix for vercel#11107 (JS modules are loaded twice). A more detailed explanation of the investigation that led to this PR can be found in the issue's comments (vercel#11107 (comment)).

Replicability

In my own project using "next": "^10.0.1" the issue is reproduced.
In the issue's comments, a developer reported a replication using "^10.0.7".
The issue is reproduced on https://nextjs.org/ by looking at the network tab, filtering by .js files and looking for pairs of identical calls with one originating from initial HTML (preload) and another one from a script (adding prefetch link to DOM).

Some information about the fix

• Both preload and prefetch values for <link rel="x"> behave similarly, with the difference being in network priority level (preload is high priority, prefetch is low priority).

• Next.js uses <link rel="preload"> in its initial HTML but then only uses <link rel="prefetch"> for the rest of the lifetime of the page.

• However, when Next.js detects that a script should be requested in advance, it only checks for matching <link rel="prefetch"> and not <link rel="preload"> (which have higher priority and are present earlier in the DOM, thus have a greater likelihood of being already loaded).

This PR aims to fix that oversight.

Potential issues (none AFAIK)

As far as I can tell by looking through the codebase, there is no downside not to add a prefetch when a preload is already in the DOM. No other script looks for a <link> based on its rel attribute.