bst artifact/source checkout UI should be made consistent with workspace checkout

Background

Currently, bst artifact checkout requires specifying a --dir or a --tar flag, and a directory/tar name. This is somewhat cumbersome (I just want my output!), and much like the situation for workspaces (#695 (closed)), should change!

Task description

bst artifact/source checkout foobar.bst should be a little smarter, and more similar to bst workspace open.

A sensible approach would be to create a directory with the name of the specified element (without the .bst suffix), and take an argument to override that name.

• For tar output, this could either infer what the desired format is from the argument or we could make --tar a switch that outputs a tarfile instead of a directory (inferring compression as with #1048 (closed)).
• We can also consider detecting piping and making our piped output a tar

I.e., I'm proposing: bst artifact checkout [--tar] <element> [<directory>]

Acceptance Criteria

I want to say bst artifact checkout [--tar] <element> [<directory>], but any UI close to bst workspace would work.

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(Newcomers: simple, but make sure you discuss this a little on IRC/the ML first, since it's not decided yet)