Consider storing remote file cache metadata in same file as text

[dsherret]

We're currently loading two files for each cached remote file.

One file contains the CachedUrlMetadata:

/// Cached metadata about a url.
#[derive(Serialize, Deserialize)]
pub struct CachedUrlMetadata {
  pub headers: HeadersMap,
  pub url: String,
  #[serde(default = "SystemTime::now")]
  pub now: SystemTime,
}

The other file contains the actual remote file source text.

We store them separately, which means we have to do two file reads per remote file when loading.

If we stored them together in the same file, then we could reduce the number of file reads we have to do from two to one per file.

This has some implications for deno info --json because we expose the "local" file. Related #17703