Support lockfile version 3

[brettwillis]

Since NPM version 7, the package-lock.json version 2 file now uses a packages field, and the dependencies field is duplicated for backwards compatibility.

It is possible to configure NPM to use a package-lock.json version 3, which omits the dependencies field to greatly reduce the size of the lock file. See https://docs.npmjs.com/cli/v8/using-npm/config#lockfile-version.

To support lockfile-version=3, patch-package must be able to interpret the packages field of the lock file.