move splitPackageNames to its own module

lib/utils/split-package-names.js

@@ -0,0 +1,23 @@
+'use strict'
+
+const splitPackageNames = (path) => {
+  return path.split('/')
+    // combine scoped parts
+    .reduce((parts, part) => {
+      if (parts.length === 0)
+        return [part]
+
+      const lastPart = parts[parts.length - 1]
+      // check if previous part is the first part of a scoped package
+      if (lastPart[0] === '@' && !lastPart.includes('/'))
+        parts[parts.length - 1] += '/' + part
+      else
+        parts.push(part)
+
+      return parts
+    }, [])
+    .join('/node_modules/')
+    .replace(/(\/node_modules)+/, '/node_modules')
+}
+
+module.exports = splitPackageNames

test/lib/utils/split-package-names.js

@@ -0,0 +1,17 @@
+'use strict'
+
+const { test } = require('tap')
+const splitPackageNames = require('../../../lib/utils/split-package-names.js')
+
+test('splitPackageNames', t => {
+  const assertions = [
+    ['semver', 'semver'],
+    ['read-pkg/semver', 'read-pkg/node_modules/semver'],
+    ['@npmcli/one/@npmcli/two', '@npmcli/one/node_modules/@npmcli/two'],
+    ['@npmcli/one/semver', '@npmcli/one/node_modules/semver'],
+  ]
+
+  for (const [input, expected] of assertions)
+    t.equal(splitPackageNames(input), expected, `split ${input} correctly`)
+  t.end()
+})