Ensure `source checkout` is symmetric to `artifact checkout`

James Ennisrequested to merge
jennis/update_source_checkout into master

Description

As described in the (very old) mailing list post: https://mail.gnome.org/archives/buildstream-list/2018-November/msg00106.html our source checkout command works very differently to artifact checkout and workspace open.

This patch is a step towards making our commands more consistent.

Instead of bst source checkout ELEMENT LOCATION we can now checkout sources with bst source checkout ELEMENT or bst source checkout --directory $dir ELEMENT.

Additionally, the --compression option (which is supported by bst artifact checkout) has been added as a possible option for bst source checkout.

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