fix(pkglock): readPkgLock loop stops

[fabienjuif]

Hi 👋

I did run tink in a project folder without package-lock.json, it resulted in an infinite loop.
After digging around, I found where the problem was (and this is what this PR fixes).

Please read the commit description if you need some context about the modifications.

Before this PR

1. there is no package-lock.json
2. I run tink
3. tink hang and never responds

➜  project git:(master) ✗ tink -l silly sh ./src/index.js
tink verb prepare checking package-lock is up to date.

After this PR

1. there is no package-lock.json
2. I run tink
3. tink create a new package-lock.json and try to run ./src/index.js

➜  project git:(master) ✗ tink -l silly sh ./src/index.js
tink verb prepare checking package-lock is up to date.
tink verb prepare Fetching and installing dependencies.
tink info prepare initializing installer
tink verb prepare installation prefix: /Volumes/sensitive/work/tink-pnp/demo/project
tink verb checkLock verifying package-lock data
tink verb teardown shutting down
tink info teardown Done in 0.001s
[...]

Have a nice weekend,