feat: add option to declare an included Taskfile as flatten (#1704)

testdata/includes_flatten/.gitignore

@@ -0,0 +1 @@
+*.txt

testdata/includes_flatten/Taskfile.multiple.yml

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+version: '3'
+
+includes:
+  included:
+    taskfile: ./included
+    flatten: true
+
+tasks:
+  gen:
+    cmds:
+      - echo "gen multiple"
+

testdata/includes_flatten/Taskfile.yml

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+version: '3'
+
+includes:
+  included:
+    taskfile: ./included
+    dir: ./included
+    flatten: true
+
+tasks:
+  default:
+    cmds:
+      - echo root_directory > root_directory.txt
+

testdata/includes_flatten/included/Taskfile.yml

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+version: '3'
+
+includes:
+    nested:
+      taskfile: ../nested
+      flatten: true
+
+tasks:
+  gen:
+    cmds:
+      - echo "gen from included"
+
+  with_deps:
+    deps:
+        - gen
+    cmds:
+      - echo "with_deps from included"
+
+
+  pwd:
+    desc: Print working directory
+    cmds:
+      - pwd

testdata/includes_flatten/nested/Taskfile.yml

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+version: '3'
+
+tasks:
+  from_nested:
+    cmds:
+      - echo "from nested"