Rearrange the project structure

[RobAtticus]

We are making a push to make our projects eventually all conform
with Golang best practices and this project setup seems to be widely
agreed upon in the community.

For now I have moved all packages into internal/; however, we may
find that some of them are useful to expose to other projects, at
which point we should move them to pkg/. All the files related to
the 'outflux' binary itself are now located in cmd/outflux.

A small bug fix is also included in schema_transfer.go, where the
wrong error message was being printed.

[RobAtticus]

I am interested in hearing others thoughts on this directory structure. I used it for timescaledb-tune and moved timescaledb-parallel-copy to it. I know we have a couple of CLI applications we are maintaining now, so it would be nice as an org to come to consensus.

This seems to be a "community standard" but I think Cobra does things slightly differently. So happy to hear other thoughts.

[LeeHampton]

I like this approach. I was using pkg but making it internal unless exposed makes more sense. Cobra does have a weird recommended way of doing things where each subcommand is a package or something, but there are several issues complaining about it: spf13/cobra#641 ... I think it's fine to just have all of the CLI/cobra stuff as part of the main package.