Make bst help work

Description

As a first time user of buildstream I blindly run bst help in the hope that it provides some useful getting started information.

Previously it would print:

$ bst help
Usage: bst [OPTIONS] COMMAND [ARGS]...

Error: No such command "help".

which was rather unhelpful.

This MR introduces the following behaviour:

bst help
Usage: bst [OPTIONS] COMMAND [ARGS]...

  Build and manipulate BuildStream projects

  Most of the main options override options in the user preferences
  configuration file.
...

CHANGELOG/Release Notes

• Add help command

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