fix(unstable): vendor cache should support adding files to hashed directories (#20070)

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
```

0 files changed, 0 insertions, 0 deletions