Add a CI check to automatically verify the checksums of newly added
package versions:
- [x] a new command, `spack ci verify-versions`
- [x] a GitHub actions check to run the command
- [x] tests for the new command
This also eliminates the suggestion for maintainers to manually verify added
checksums in the case of accidental version <--> checksum mismatches.
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Signed-off-by: Todd Gamblin <tgamblin@llnl.gov>
