Plugin loading refactor, removing all versioning

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.

1 files changed, 0 insertions, 21 deletions

diff --git a/src/buildstream/plugin.py b/src/buildstream/plugin.py
index 2182f5a08..4f2d74304 100644
--- a/src/buildstream/plugin.py
+++ b/src/buildstream/plugin.py
@@ -148,27 +148,6 @@ class Plugin:
         which are included in the buildstream namespace.
     """
 
-    BST_REQUIRED_VERSION_MAJOR = 0
-    """Minimum required major version"""
-
-    BST_REQUIRED_VERSION_MINOR = 0
-    """Minimum required minor version"""
-
-    BST_FORMAT_VERSION = 0
-    """The plugin's YAML format version
-
-    This should be set to ``1`` the first time any new configuration
-    is understood by your :func:`Plugin.configure() <buildstream.plugin.Plugin.configure>`
-    implementation and subsequently bumped every time your
-    configuration is enhanced.
-
-    .. note::
-
-       Plugins are expected to maintain backward compatibility
-       in the format and configurations they expose. The versioning
-       is intended to track availability of new features only.
-    """
-
     BST_PLUGIN_DEPRECATED = False
     """True if this element plugin has been deprecated.