This feature generates a verification manifest for each installed
package and provides a command, "spack verify", which can be used to
compare the current file checksums/permissions with those calculated
at installed time.
Verification includes
* Checksums of files
* File permissions
* Modification time
* File size
Packages installed before this PR will be skipped during verification.
To verify such a package you must reinstall it.
The spack verify command has three modes.
* With the -a,--all option it will check every installed package.
* With the -f,--files option, it will check some specific files,
determine which package they belong to, and confirm that they have
not been changed.
* With the -s,--specs option or by default, it will check some
specific packages that no files havae changed.