fix(sveltekit): Ensure target file exists before applying auto instrumentation

[Lms24]

In our auto instrumentation Vite plugin for SvelteKit, we read +(page|layout)(.server).(js|ts) files' code to determine if we should add our wrapper to the file or not. We previously didn't check for a file id's existence before reading the file if the id matched that certain pattern, wrongly assuming that these ids would always map to actually existing files.

It seems like Vite plugins such as Houdini's plugin add file ids to the build for files that actually don't exist (#8846, #8854) . When our plugin's load hook was called for such an id, it then tried to access and read the file which caused a build error.

This patch now adds a file existence guard to ensure we simply no-op for these files.

(Wild guess: Houdini adds load functions for all (some?) routes so that it can do its graph QL magic under the hood. Meaning, if we no-op at build time, these functions might not be instrumented with Sentry spans. It's probably not the end of the world as errors should still be caught by the handleError hook. But let's see. I already cc'd the Houdini maintainers in #8846 to let them know of this).

closes #8846
closes #8854

[lforst]

ah yes virtual modules