fix(packument): eliminate _cached field (#175)

BREAKING CHANGE: the `_cached` attribute has been removed from packuments.

README.md

@@ -276,7 +276,6 @@ For Pacote's purposes, the following fields are relevant:
 * `time` In the full packument, an object mapping version numbers to
   publication times, for the `opts.before` functionality.
 
-Pacote adds the following fields, regardless of the accept header:
+Pacote adds the following field, regardless of the accept header:
 
-* `_cached` Whether the packument was fetched from the network or the local cache.
 * `_contentLength` The size of the packument.

lib/registry.js

@@ -97,7 +97,6 @@ class RegistryFetcher extends Fetcher {
         integrity: null,
       })
       const packument = await res.json()
-      packument._cached = res.headers.has('x-local-cache')
       packument._contentLength = +res.headers.get('content-length')
       if (this.packumentCache) {
         this.packumentCache.set(this.packumentUrl, packument)