[BREAKING] Make '@ui5/builder' an optional peerDependency

This decoupling allows the use of @ui5/project without having to install
@ui5/builder with all its transitive dependencies in cases where no
build functionality is required. An example for that is UI5 linter.

BREAKING CHANGE:
Consumers of the Node.js API that make use of the ProjectGraph#build
method need to declare a dependency to '@ui5/builder' in their
respective package.json. The package manager should ensure that the
version fullfills the range specified in the package.json of
'@ui5/project'.

lib/graph/ProjectGraph.js

@@ -513,7 +513,13 @@ class ProjectGraph {
 
 	async _getTaskRepository() {
 		if (!this._taskRepository) {
-			this._taskRepository = await import("@ui5/builder/internal/taskRepository");
+			try {
+				this._taskRepository = await import("@ui5/builder/internal/taskRepository");
+			} catch (err) {
+				throw new Error(
+					`Failed to load task repository. Missing dependency to '@ui5/builder'? ` +
+					`Error: ${err.message}`);
+			}
 		}
 		return this._taskRepository;
 	}