fix: Limit concurrent open files during lsstream (#285)

<!-- What / Why -->
<!-- Describe the request in detail. What it does and why it's being
changed. -->
During 'npm cache verify', currently all the cache files are open at the
same time, which will bring EMFILE error in an environment that limit
max open files.
This change limits the concurrent open files in garbageCollect() with
p-map module to avoid this problem.
I first sent this pull request to npm/cli and it was merged, but
realized that the original was in this repository.

## References
npm/cli#7631
npm/cli#4783

lib/entry-index.js

@@ -19,6 +19,9 @@ const hashToSegments = require('./util/hash-to-segments')
 const indexV = require('../package.json')['cache-version'].index
 const { moveFile } = require('@npmcli/fs')
 
+const pMap = require('p-map')
+const lsStreamConcurrency = 5
+
 module.exports.NotFoundError = class NotFoundError extends Error {
   constructor (cache, key) {
     super(`No cache entry for ${key} found in ${cache}`)
@@ -182,15 +185,15 @@ function lsStream (cache) {
   // Set all this up to run on the stream and then just return the stream
   Promise.resolve().then(async () => {
     const buckets = await readdirOrEmpty(indexDir)
-    await Promise.all(buckets.map(async (bucket) => {
+    await pMap(buckets, async (bucket) => {
       const bucketPath = path.join(indexDir, bucket)
       const subbuckets = await readdirOrEmpty(bucketPath)
-      await Promise.all(subbuckets.map(async (subbucket) => {
+      await pMap(subbuckets, async (subbucket) => {
         const subbucketPath = path.join(bucketPath, subbucket)
 
         // "/cachename/<bucket 0xFF>/<bucket 0xFF>./*"
         const subbucketEntries = await readdirOrEmpty(subbucketPath)
-        await Promise.all(subbucketEntries.map(async (entry) => {
+        await pMap(subbucketEntries, async (entry) => {
           const entryPath = path.join(subbucketPath, entry)
           try {
             const entries = await bucketEntries(entryPath)
@@ -213,9 +216,12 @@ function lsStream (cache) {
             }
             throw err
           }
-        }))
-      }))
-    }))
+        },
+        { concurrency: lsStreamConcurrency })
+      },
+      { concurrency: lsStreamConcurrency })
+    },
+    { concurrency: lsStreamConcurrency })
     stream.end()
     return stream
   }).catch(err => stream.emit('error', err))