Fix decoding lock files with path dependencies

Fix decoding lock files with path dependencies

With the previous changes a path dependency must have the precise path to it
listed in its package id. Currently when decoding a lockfile, however, all path
dependencies have the same package id, which unfortunately causes a mismatch.

This commit alters the decoding of a lockfile to perform some simple path
traversals to probe the filesystem to understand where path dependencies are and
set the right package id for the found packages.