Add a method in bp2build to create aliases in another directory

The expected use case for this is to create aliases for stub libraries
in the @api_surfaces repository in build/bazel/api_surfaces.

This restricts the scope to just aliases. If we have a use case for
generating actual Bazel targets in another directory, a workaround could
be to generate the targets in the current directory (via
CreateBazelTargetModule) and aliases to it in the other directory

Test: go test ./bp2build
Change-Id: I6b63d9d018618d447fc7c260a2a94aaa00e57a4d

android/mutator.go

@@ -268,6 +268,11 @@ type TopDownMutatorContext interface {
 	// platforms, as dictated by a given bool attribute: the target will not be buildable in
 	// any platform for which this bool attribute is false.
 	CreateBazelTargetModuleWithRestrictions(bazel.BazelTargetModuleProperties, CommonAttributes, interface{}, bazel.BoolAttribute)
+
+	// CreateBazelTargetAliasInDir creates an alias definition in `dir` directory.
+	// This function can be used to create alias definitions in a directory that is different
+	// from the directory of the visited Soong module.
+	CreateBazelTargetAliasInDir(dir string, name string, actual bazel.Label)
 }
 
 type topDownMutatorContext struct {
@@ -705,6 +710,34 @@ func (t *topDownMutatorContext) CreateBazelTargetModuleWithRestrictions(
 	t.createBazelTargetModule(bazelProps, commonAttrs, attrs, enabledProperty)
 }
 
+var (
+	bazelAliasModuleProperties = bazel.BazelTargetModuleProperties{
+		Rule_class: "alias",
+	}
+)
+
+type bazelAliasAttributes struct {
+	Actual *bazel.LabelAttribute
+}
+
+func (t *topDownMutatorContext) CreateBazelTargetAliasInDir(
+	dir string,
+	name string,
+	actual bazel.Label) {
+	mod := t.Module()
+	attrs := &bazelAliasAttributes{
+		Actual: bazel.MakeLabelAttribute(actual.Label),
+	}
+	info := bp2buildInfo{
+		Dir:             dir,
+		BazelProps:      bazelAliasModuleProperties,
+		CommonAttrs:     CommonAttributes{Name: name},
+		ConstraintAttrs: constraintAttributes{},
+		Attrs:           attrs,
+	}
+	mod.base().addBp2buildInfo(info)
+}
+
 // ApexAvailableTags converts the apex_available property value of an ApexModule
 // module and returns it as a list of keyed tags.
 func ApexAvailableTags(mod Module) bazel.StringListAttribute {