This new test replaces the test in tests/format/project.py.

The BST_MIN_VERSION guards assert that the BuildStream core which
loaded the plugin is compatible with the plugin itself.

This commit adds BST_MIN_VERSION to the base plugin.py with documentation
informing Plugin authors how to set the minimum version, and also adds
the assertions at plugin loading time in pluginfactory.py.

This commit also:

  * Adds the BST_MIN_VERSION specification to all current core plugins
  * Adds the BST_MIN_VERSION specification to plugins used in test cases

Plugin format versioning was decided to be removed for local
plugins and any plugins for which we do not load an explicitly
provided plugin. For pip, this will be handled with a standard
distutils/setuptools approach, allowing users to specify pip
style version boundaries in the plugin origins.

This patch refactors plugin loading so that all related code
goes into the private _pluginfactory module, a new small
PluginOrigin type was added to better manipulate loaded
origins.

Test cases have been removed and will be readded in a following
commit, adjusted to new expectations.

  * "min-version" is specified as a <major>.<minor> point version
    and uses the installed BuildStream version instead of having
    a separate versioning number for the format.

  * The presence of "format-version" is now used to indicate
    that we might be loading a BuildStream 1 project.

  * For now, where parsing the version at startup is concerned, and
    also where `bst init` is concerned, we artificially bump the
    detected BuildStream version to 2.0 if we detect a version < 2.0,
    these exceptions can be removed once 2.0 is tagged and released.

Summary of changes:

  _project.py: Now parse "min-version" and detect "format-version" to
               warn about loading a BuildStream 1 project

  _versions.py: Remove obsolete BST_FORMAT_VERSION numbers from here

  data/projectconfig.yaml: Remove old "format-version" from defaults

  utils.py: Added new private _parse_version() helper function, and another
            _get_bst_api_version() to get an adjusted API version.

  frontend/app.py, frontend/cli.py: Updated `bst init` implementation

  testing (buildstream.testing): Updated testing utilities to generate
  and use projects with min-version instead of format-version.

  tests and examples: Updated to use min-version across the board.

This is not needed now that we have 'is_resolved' and 'is_cached'.
We can therefore drop all calling places and implementations of it.

As discussed over the mailing list, reformat code using Black. This is a
one-off change to reformat all our codebase. Moving forward, we
shouldn't expect such blanket reformats. Rather, we expect each change
to already comply with the Black formatting style.

These also assert that the provenance of the references to missing
files are reported, guarding for regressions of issue #947

This is where other load time related plugin error handling is
checked, and is the last thing to remove in the `tests/pipeline`
directory.

The tests/format/project.py test already has some tests about how
we error gracefully for bad plugins and bad plugin configurations,
lets put it there rather than tests/pipeline/load.py which we
will remove.

This adds a regression test for issue #803