Define build time dependencies (#181)

* Define build time dependencies

* Update changelog

* Address PR comments

* Extract to deps.yaml

* Rename deps.yml to build.yml

* Move build.yml under _dev/build

* Replace array with obj

* Fix

test/packages/good/_dev/build/build.yml

@@ -0,0 +1,3 @@
+dependencies:
+  ecs:
+    reference: git@7a11f3e1f7d8f05e8f06623fd98254b8cb34c441

versions/1/_dev/build/build.spec.yml

@@ -0,0 +1,26 @@
+##
+## Describes the specification for the package's main manifest.yml file
+##
+spec:
+  # Everything under here follows JSON schema (https://json-schema.org/), written as YAML for readability
+  type: object
+  additionalProperties: false
+  properties:
+    dependencies:
+      type: object
+      description: Package dependencies
+      additionalProperties: false
+      properties:
+        ecs:
+          description: ECS dependency
+          type: object
+          additionalProperties: false
+          properties:
+            reference:
+              type: string
+              description: Source reference
+              pattern: 'git@[a-z0-9]{40}'
+          required:
+            - reference
+  required:
+  - dependencies

versions/1/_dev/build/spec.yml

@@ -1,6 +1,11 @@
 spec:
   additionalContents: false
   contents:
+    - description: Package build manifest
+      type: file
+      contentMediaType: "application/x-yaml"
+      name: "build.yml"
+      $ref: "./build.spec.yml"
     - description: Folder containing resources related to docs.
       type: folder
       name: docs

versions/1/changelog.yml

@@ -132,4 +132,7 @@
     link: https://github.com/elastic/package-spec/pull/183
   - description: Introduce "reference" property for fields
     type: enhancement
-    link: https://github.com/elastic/package-spec/pull/179
+    link: https://github.com/elastic/package-spec/pull/179
+  - description: Define build-time dependencies
+    type: enhancement
+    link: https://github.com/elastic/package-spec/pull/181