Create new PluginResolver

[mason-bially]

Besides moving the existing one out of system.py, a new one should be created with a behaviour like system.

Specifically it should allow for the addition of rules like:

• PreferDirectory(directory, filter=identity, reverse=False) -- Where directory is a PluginDirectory object and filter is a callable which takes the list of all plugin directories and returns the ones to which the rule applies. Reverse causes the result of filter to be prefer over this one.
• PreferImplementation(implementation_name, filter=idetity, plugin_filter=identity, reverse=False) -- Prefers a specific implementation over all others. Filter filters implementation names that this rule applies to. plugin_filter filters plugin_ids that this rule applies too. (Note that filter and plugin_filter should probably return similar values). reverse again reverses the preference.
• PreferVersion(version, filter=identity, plugin_filter=identity, reverse=False) -- Like above but with versions
• OrderImplementations(order_function, plugin_filter=identity) -- Gives order_function the list of implementations and asks it to order them by preference. Plugin filter, as always, filters which plugins this applies to.
• OrderVersions(order_function, plugin_filter=identity) -- Gives order_function the list of versions and asks it to order them by preference. Plugin filter, as always, filters which plugins this applies to.
• Ignore(function) -- Ignores any plugins matching function.
• Alias(aliased_name, original_name) -- Creates an alias for an existing namespace. For example: Alias("thing.current", "thing.bird") would alias an lookup of "thing.current" to instead resolve as "thing.bird". All aliases are absolute.
• Maybe more.

Some helper objects would be useful:

• NamespaceFilter
• VersionFilter
• ImplementationFilter
• DirectoryFilter

Also the plugin resolver should have a couple default rules in the following priority order:

• "madz_prefer_internal": PreferDirectory()
• "madz_order_semver": OrderVersions per semver
• "madz_prefer_default": PreferImplementation("")
• "madz_order_alphabetical": OrderImplementations per alphabetical

The PluginResolver should allow rules to be labled for ease of addition/insertion/removal. It will likely have to use some pruning mechanisms, and should follow rules sequentially. It's job is twofold given a plugin_id:

• Complete missing information.
• Choose a plugin that best fits the resolution order. It may sometimes be asked for multiple plugins, or all matching plugins.

[cgrinker]

dd5a9b6388b1c5a26b90198ec80b706dcdb86ccb

[mason-bially]

In hindsight, my original descriptions were too specific, the following general operations would be nice to have:

• Alias - Generate additional possible matches using an alias.
• Filter/Select - Prune/choose possibilities that match some condition.
• Order - Order possibilities.