Support multiple ./packages-*/ directories

This line: https://github.com/Shopify/shipit-engine/blob/740b8a98eb7c11f84378334270e320d986ce6b1a/lib/snippets/assert-lerna-independent-version-tags#L19

Fails deploys of polaris-icons for packages not in /packages/.

In this deployment:
https://shipit.shopify.io/shopify/polaris-icons/libraries-js/deploys/843212

The version in ./packages-ruby/polaris_icons/package.json isn't being detected correctly:

$ assert-lerna-independent-version-tags
pid: 362510
module.js:550
    throw err;
    ^
Error: Cannot find module './packages/polaris_icons/package.json'
    at Function.Module._resolveFilename (module.js:548:15)
    at Function.Module._load (module.js:475:25)
    at Module.require (module.js:597:17)
    at require (internal/module.js:11:18)
    at [eval]:1:13
    at ContextifyScript.Script.runInThisContext (vm.js:50:33)
    at Object.runInThisContext (vm.js:139:38)
    at Object.<anonymous> ([eval]-wrapper:6:22)
    at Module._compile (module.js:653:30)
    at evalScript (bootstrap_node.js:479:27)
polaris_icons is tagged as 0.3.0, but its package.json version is .

The relevant entry point is lib/snippets/assert-lerna-independent-version-tags, and the failure is reproduced by running assert-lerna-independent-version-tags against the polaris-icons layout. Trace how ./packages/.../package.json is selected, then verify that package.json files under packages-* are detected and their versions are compared with tags.