Improve message API

Background

Currently, the message API in _context.py is cumbersome to use, and requires fiddling with a Message and MessageType object. This has lead to several re-implementations as helpers in various other locations in the codebase.

It also makes writing quick debug statements hard, and encourages raising exceptions for debugging.

This is annoying!

Task description

We should improve the message API. A first step could be to add helpers such as info(), error(), warn() to _context.py - this would already help make debugging easier, and be closer to what various logging libraries provide (no need to figure out how to instantiate a Message again).

We can then think about improving the (currently, mostly unused) --debug flag to support filtering messages by specific domains.

Excerpt from an IRC discussion on the topic:

<tristan> tlater, I'd like to launch:   `bst --debug=sandbox:compose-element build ...`
<tristan> tlater, like that                                                                                                                         [16:47]
<tristan> and then get debug messages in the logs from the active sandbox implementation, and from statements in the compose element

Acceptance Criteria

We should have a nicer messaging API to make debugging simpler.

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