extension_cli: Properly populate manifest with snippet location (#44425)

This fixes an issue where the snippet file location would not be the
proper one for compiled extensions because it would be populated with an
absolute path instead of a relative one in relation to the extension
output directory. This caused the copy operation downstream to not do
anything, because it copied the file to the location it already was
(which was not the output directory for that extension).

Also adds some tests and pulls in the `Fs` so we do not have such issues
with snippets a third time hopefully.

Release Notes:

- N/A

crates/extension_cli/src/main.rs

@@ -71,6 +71,7 @@ async fn main() -> Result<()> {
             &extension_path,
             &mut manifest,
             CompileExtensionOptions { release: true },
+            fs.clone(),
         )
         .await
         .context("failed to compile extension")?;