`bst build` should add .bst on the file if it's missing

[See original issue on GitLab](https://gitlab.com/BuildStream/buildstream/-/issues/1358)
In GitLab by [[Gitlab user @tcanabrava]](https://gitlab.com/tcanabrava) on Jul 8, 2020, 11:47

Quite a few times I was typing bst build dolphin instead of `bst build dolphin.bst`, considering that we can think as the elements as "targets", in the make sense, it's good to have it behave like a target too.

Start at the `bst build` command entry point and trace how its target argument is resolved. Verify the behavior with an element name both with and without `.bst`; done means `bst build dolphin` resolves the same target as `bst build dolphin.bst`.

Written by the indexing model from the issue text.