Add warning when include is used in config files other than databricks.yml (#2389)

## Changes
Defining an include section in config files other than the main
`databricks.yml` file fails silently. With this PR users will get a
warning when they try this.

## Tests
Acceptance test.

acceptance/bundle/includes/include_outside_root/a.yml

@@ -0,0 +1,4 @@
+include:
+  - b.yml
+  - c.yml
+

acceptance/bundle/includes/include_outside_root/databricks.yml

@@ -0,0 +1,5 @@
+bundle:
+  name: include_outside_root
+
+include:
+ - a.yml

acceptance/bundle/includes/include_outside_root/output.txt

@@ -0,0 +1,16 @@
+Warning: Include section is defined outside root file
+  at include
+  in a.yml:2:3
+
+The include section is defined in a file that is not the root file.
+These values will be ignored because only the includes defined in
+the bundle root file (that is databricks.yml or databricks.yaml)
+are loaded.
+
+Name: include_outside_root
+Target: default
+Workspace:
+  User: [USERNAME]
+  Path: /Workspace/Users/[USERNAME]/.bundle/include_outside_root/default
+
+Found 1 warning

acceptance/bundle/includes/include_outside_root/script

@@ -0,0 +1 @@
+$CLI bundle validate

bundle/config/loader/process_include.go

@@ -161,6 +161,19 @@ func (m *processInclude) Apply(_ context.Context, b *bundle.Bundle) diag.Diagnos
 		return diags
 	}
 
+	if len(this.Include) > 0 {
+		diags = diags.Append(diag.Diagnostic{
+			Severity: diag.Warning,
+			Summary:  "Include section is defined outside root file",
+			Detail: `The include section is defined in a file that is not the root file.
+These values will be ignored because only the includes defined in
+the bundle root file (that is databricks.yml or databricks.yaml)
+are loaded.`,
+			Locations: this.GetLocations("include"),
+			Paths:     []dyn.Path{dyn.MustPathFromString("include")},
+		})
+	}
+
 	err := b.Config.Merge(this)
 	if err != nil {
 		diags = diags.Extend(diag.FromErr(err))