fix: fixed dependency bug & simplified TaskGraph

Fixes #386.

This bug was fixed here as a side-effect of simplifying the
implementation of TaskGraph's dependency logic.

Previously, dependency relationships of nodes in TaskGraph were updated
by adding/removing from the dependencies/dependants arrays of the
appropriate TaskNodes when nodes were added or removed.

This logic was somewhat opaque and difficult to maintain/debug. Here, we
change the implementation to simply rebuild the dependency relationships
of all nodes in TaskGraph's index whenever a node is added or removed.

With this implementation, we no longer need to track dependants or handle
predecessors (since addTaskInternal and processTasksInternal are never
mutually concurrent), and the logic is hopefully simpler and easier to
maintain.

garden-service/src/commands/dev.ts

@@ -116,7 +116,7 @@ export class DevCommand extends Command<Args, Opts> {
           fromWatch: watch,
           hotReloadServiceNames,
           force: watch,
-          forceBuild: watch,
+          forceBuild: false,
           includeDependants: watch,
         }))
       }