These flags don't make sense in the checkMigrations command I'm
currently writing.

We may wish to, in the future, but anything that requires all these
params in one sub-package to specify the params once.

Preliminary. This requires more testing, command line params and
integration with CI.

To make it possible to test that little filter I had to add an
interface (fine) but a bunch of code to copy slice of objects into a
slice of interfaces (less fine). To make the GitHubfile structure match
the NamedFile interface I had to make its Name field lowercase (private). Also
fine, but then that meant it didn't marshal from JSON, so I had to add
an internal struct with exported fields. Whew.

And test it.

Testing revealed I was mutating in place, which I did not want to do, so hooray!

You still have to point at Github though

And the new migrations

Anything that isn't shared is moved to backfill.go. This is kind of
misleading since all the code is in the same package anyway, but as part
of this I did need to move the PreRunPersistent in checkMigrationsCmd to
be PreRun so that both the logging and initGithubParams functions are called.

Add validatemigrationorder to Makefile. The name checkmigrations has other,
similar names in the Makefile related to generating migrations, so don't
use that.

Add an executable target to the Makefile, and depend on that for
validatemigrationorder. This is more make appropriate.

Make the build task always rebuild the executable, to match existing behavior.

Use wrapped errors for clarification, and use differently errors instead
of reusing err.