fix searched paths in DataLoader.path_dwim_relative (avoid AnsibleFileNotFound) (#26729)

* add unit test: nested dynamic includes

* nested dynamic includes: avoid AnsibleFileNotFound error

Error was:
Unable to retrieve file contents
Could not find or access 'include2.yml'

Before 8f758204cf, at the end of
'path_dwim_relative' method, the 'search' variable contained amongst
others paths:
'/tmp/roles/testrole/tasks/tasks/included.yml' and
'/tmp/roles/testrole/tasks/included.yml'.
The commit mentioned before removed the last one despite the method
docstrings specify 'with or without explicitly named dirname subdirs'.

* add integration test: nested includes

test/integration/targets/includes/roles/test_includes/tasks/branch_toplevel.yml

@@ -0,0 +1,9 @@
+# 'canary2' used instead of 'canary', otherwise a "recursive loop detected in
+# template string" occurs when both includes use static=yes
+- include: 'leaf_sublevel.yml canary2={{ canary }}'
+  static: yes
+  when: 'nested_include_static|bool' # value for 'static' can not be a variable, hence use 'when'
+
+- include: 'leaf_sublevel.yml canary2={{ canary }}'
+  static: no
+  when: 'not nested_include_static|bool'

test/integration/targets/includes/roles/test_includes/tasks/leaf_sublevel.yml

@@ -0,0 +1,2 @@
+- set_fact:
+    canary_fact: '{{ canary2 }}'

test/integration/targets/includes/roles/test_includes/tasks/main.yml

@@ -80,3 +80,27 @@
     # both these via a handler include
     - included_handler
     - verify_handler
+
+- include: branch_toplevel.yml canary=value1 nested_include_static=no
+  static: no
+- assert:
+    that:
+      - 'canary_fact == "value1"'
+
+- include: branch_toplevel.yml canary=value2 nested_include_static=yes
+  static: no
+- assert:
+    that:
+      - 'canary_fact == "value2"'
+
+- include: branch_toplevel.yml canary=value3 nested_include_static=no
+  static: yes
+- assert:
+    that:
+      - 'canary_fact == "value3"'
+
+- include: branch_toplevel.yml canary=value4 nested_include_static=yes
+  static: yes
+- assert:
+    that:
+      - 'canary_fact == "value4"'