Consistency check for Julia depot

[vchuravy]

Occasionally we run into scenarios where it would be handly to have a way to check for the consistency of a package environment.

As an example:

1. Package installation fails due to out of disk space, leaves inconsistent state and leads to fun errors (the one I recall was a syntax error during package precompilation, that a end was missing, because the file was truncated in the middle)
2. HPC clusters have eviction policies... Often the Julia depot is on a fast scratch environment, but some HPC centers evict files that are not accessed for a period, leaving the depot in an inconsistent state. As an example: https://docs.cscs.ch/storage/filesystems/#cleanup-and-expiration_1

The user then get's a "ERROR: failed to find source of parent package: "Accessors"" which gives a hint that the depot is messed up.