Add a function to create config_setting(s)

The use case for this is creating config_setting(s) specific to an apex
variant and selecting stub/impl in that config_setting. We likely need
only a handful of such config_setting(s), but determining that list
requires iterating the build graph.

Test: go test ./bp2build
Change-Id: I9aa552e3d0bcf67513023c3a7d4bbf8fae464ee4

bp2build/testing.go

@@ -317,6 +317,8 @@ type customProps struct {
 	One_to_many_prop *bool
 
 	Api *string // File describing the APIs of this module
+
+	Test_config_setting *bool // Used to test generation of config_setting targets
 }
 
 type customModule struct {
@@ -490,6 +492,27 @@ func (m *customModule) ConvertWithBp2build(ctx android.TopDownMutatorContext) {
 	}
 
 	ctx.CreateBazelTargetModule(props, android.CommonAttributes{Name: m.Name()}, attrs)
+
+	if proptools.Bool(m.props.Test_config_setting) {
+		m.createConfigSetting(ctx)
+	}
+
+}
+
+func (m *customModule) createConfigSetting(ctx android.TopDownMutatorContext) {
+	csa := bazel.ConfigSettingAttributes{
+		Flag_values: bazel.StringMapAttribute{
+			"//build/bazel/rules/my_string_setting": m.Name(),
+		},
+	}
+	ca := android.CommonAttributes{
+		Name: m.Name() + "_config_setting",
+	}
+	ctx.CreateBazelConfigSetting(
+		csa,
+		ca,
+		ctx.ModuleDir(),
+	)
 }
 
 var _ android.ApiProvider = (*customModule)(nil)