fix(unstable): vendor cache should support adding files to hashed directories

[dsherret]

This changes the design of the manifest.json file to have a separate "folders" map for mapping hashed directories. This allows, for example, to add files in a folder like http_localhost_8000/#testing_5de71/ and have them be resolved automatically as long as their remaining components are identity-mappable to the file system (not hashed). It also saves space in the manifest.json file by only including the hashed directory instead of each descendant file.

// manifest.json
{
  "folders": {
    "https://localhost/NOT_MAPPABLE/": "localhost/#not_mappable_5cefgh"
  },
  "modules": {
    "https://localhost/folder/file": {
      "headers": {
        "content-type": "application/javascript"
      }
    },
  }
}

// folder structure
localhost
  - folder
    - #file_2defn (note: I've made up the hashes in these examples)
  - #not_mappable_5cefgh
    - mod.ts
    - etc.ts
    - more_files.ts

[dsherret]

This property was also removed because it's unnecessary. We can derive it from the url and headers.

[bartlomieju]

LGTM

[bartlomieju]

I assume it's not possible to have a URL that has multiple hashes?

[dsherret]

There could be multiple hashed components. This selects the deepest hash in order to get the reverse mapping from there.