[fix] strip paths.base before resolving assets in preview server.

[Karlinator]

The preview server was implicitly assuming that asset URLs would never start with the base path. In fact, they always do.

Vaguely related to #2230 (but this was a new issue I discovered and diagnosed while working on #2407).

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[benmccann]

@Karlinator can you rebase this PR?

[Karlinator]

Sure thing!

Prettier is being weird for me right now. It doesn't agree with itself between the CI Lint and running lint or format locally, but I figured it out eventually.

[Karlinator]

I've noticed static assets are available both at /base-path/_app/whatever and at /_app/whatever. I can add a check to reject with 404 all requests that don't start with the base path, but looking at this I suspect there are more errors in the if/else stuff around 110? Or at the very least it's confusing as it is now. I'll have a little more of a think.

[benmccann]

Hmm. Honestly, I'd love to just get rid of all this stuff and call Vite's preview functionality. It looks like their preview method is not exposed though, so that would be the first step

[Karlinator]

I mean I don't disagree. But I think the base path stuff would still be a problem (we apparently have bugs in the dev server about this, see #2465 as you know). However, right now we are basically supporting two quite separate and different servers for preview and dev which isn't great.

[benmccann]

Yeah, if there are issues in the dev server that's probably the more important one to fix since it's more used and will stick around longer. This maybe wouldn't be worth as much effort if the code were all about to be rewritten, but that would depend on vitejs/vite#5014 and I don't know how long that'd take or if it will even be accepted, so probably worth getting this in still

[Karlinator]

I won't spend any more time trying to improve/clean up this until we know the outcome of vitejs/vite#5014 then. Hopefully we can just rip this all out soon-ish.

[raduab]

I am getting the following error when building, starting with version 171:

> Using @sveltejs/adapter-node
 > .svelte-kit/output/server/chunks/app-0d4ff97d.js:1703:25: error: Could not resolve "./global-61357231.js"
    1703 │       load: () => import("./global-61357231.js")
         ╵                          ~~~~~~~~~~~~~~~~~~~~~~

 > .svelte-kit/output/server/chunks/__layout-048eba31.js:377:7: error: Could not resolve "./global-61357231.js"
    377 │ import("./global-61357231.js");
        ╵        ~~~~~~~~~~~~~~~~~~~~~~

> Build failed with 2 errors:
.svelte-kit/output/server/chunks/__layout-048eba31.js:377:7: error: Could not resolve "./global-61357231.js"
.svelte-kit/output/server/chunks/app-0d4ff97d.js:1703:25: error: Could not resolve "./global-61357231.js"
Error: Build failed with 2 errors:
.svelte-kit/output/server/chunks/__layout-048eba31.js:377:7: error: Could not resolve "./global-61357231.js"
.svelte-kit/output/server/chunks/app-0d4ff97d.js:1703:25: error: Could not resolve "./global-61357231.js"