refactor: major overhaul to plugin architecture

Plugins are now registered and loaded separately, and are now defined
as modules with an exported `gardenPlugin` function.

BREAKING CHANGE:
This includes some changes to the project schema and how it is resolved,
as well as how the main `Garden` class is instantiated. The `Garden`
class is now called with an environment name, which is then fixed for
the session. The env configuration is resolved by merging the specific
environment configuration with a global configuration specified on the
new `global` key in the project config. The schema for the `providers`
key also different - its keys should now match plugin names, and
contain configuration for those plugins.

.nycrc

@@ -1,6 +1,10 @@
 {
   "check-coverage": true,
   "per-file": true,
+  "lines": 0,
+  "statements": 0,
+  "functions": 0,
+  "branches": 0,
   "reporter": [
     "html",
     "text"

examples/hello-world/garden.yml

@@ -1,19 +1,19 @@
 project:
   name: hello-world
+  global:
+    providers:
+      container: {}
+      npm-package: {}
+    variables:
+      my-variable: hello-variable
   environments:
     local:
       providers:
-        docker:
-          type: kubernetes
+        kubernetes:
           context: docker-for-desktop
-        gcf:
-          type: local-google-cloud-functions
+        local-google-cloud-functions: {}
     dev:
       providers:
-        docker:
-          type: google-app-engine
-        gcf:
-          type: google-cloud-functions
+        google-app-engine: {}
+        google-cloud-functions:
           default-project: garden-hello-world
-  variables:
-    my-variable: hello-variable

src/cli.ts

@@ -29,7 +29,6 @@ import { BuildCommand } from "./commands/build"
 import { EnvironmentCommand } from "./commands/environment/index"
 import { DeployCommand } from "./commands/deploy"
 import { CallCommand } from "./commands/call"
-import { defaultPlugins } from "./plugins"
 import { TestCommand } from "./commands/test"
 import { DevCommand } from "./commands/dev"
 import { LogsCommand } from "./commands/logs"
@@ -282,7 +281,7 @@ export class GardenCli {
         )
       }
 
-      const garden = await Garden.factory(root, { env, logger, plugins: defaultPlugins })
+      const garden = await Garden.factory(root, { env, logger })
       return command.action(garden.pluginContext, argsForAction, optsForAction)
     }