Introduce cc_api_library

Introduce cc_api_library, which reflects imported API from other
inner-tree. This cc_api_library module type will later refer from
the other ninja module which generates stub library from the interface
description.

Tested environment :
* original libc definition has been removed temporarily, to ensure that
  imported api stub library is being used from build
* Added new definition of libc as below
 cc_api_library {
  name: "libc",
  arch: {
    x86: {
      src: "libs/x86/libc.so",
    },
    x86_64: {
      src: "libs/x86_64/libc.so",
    },
  },
  header_libs: [
    "libc_headers",
  ],
  export_header_lib_headers: ["libc_headers"],
  min_sdk_version: "9",
  vendor_available: true,
 }

Bug: 236087698
Test: `ALLOW_MISSING_DEPENDENCIES=true m vendorimage` succeeded
Change-Id: I67070b0f3561aa2afd73b6c1c0fdf4255218baac

multitree/api_imports.go

@@ -0,0 +1,88 @@
+// Copyright 2022 Google Inc. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package multitree
+
+import (
+	"android/soong/android"
+
+	"github.com/google/blueprint"
+)
+
+var (
+	apiImportNameSuffix = ".apiimport"
+)
+
+func init() {
+	RegisterApiImportsModule(android.InitRegistrationContext)
+}
+
+func RegisterApiImportsModule(ctx android.RegistrationContext) {
+	ctx.RegisterModuleType("api_imports", apiImportsFactory)
+}
+
+type ApiImports struct {
+	android.ModuleBase
+	properties apiImportsProperties
+}
+
+type apiImportsProperties struct {
+	Shared_libs []string // List of C shared libraries from API surfaces
+	Header_libs []string // List of C header libraries from API surfaces
+}
+
+// 'api_imports' is a module which describes modules available from API surfaces.
+// This module is required to get the list of all imported API modules, because
+// it is discouraged to loop and fetch all modules from its type information. The
+// only module with name 'api_imports' will be used from the build.
+func apiImportsFactory() android.Module {
+	module := &ApiImports{}
+	module.AddProperties(&module.properties)
+	android.InitAndroidModule(module)
+	return module
+}
+
+func (imports *ApiImports) GenerateAndroidBuildActions(ctx android.ModuleContext) {
+	// ApiImport module does not generate any build actions
+}
+
+type ApiImportInfo struct {
+	SharedLibs, HeaderLibs map[string]string
+}
+
+var ApiImportsProvider = blueprint.NewMutatorProvider(ApiImportInfo{}, "deps")
+
+// Store module lists into ApiImportInfo and share it over mutator provider.
+func (imports *ApiImports) DepsMutator(ctx android.BottomUpMutatorContext) {
+	generateNameMapWithSuffix := func(names []string) map[string]string {
+		moduleNameMap := make(map[string]string)
+		for _, name := range names {
+			moduleNameMap[name] = name + apiImportNameSuffix
+		}
+
+		return moduleNameMap
+	}
+
+	sharedLibs := generateNameMapWithSuffix(imports.properties.Shared_libs)
+	headerLibs := generateNameMapWithSuffix(imports.properties.Header_libs)
+
+	ctx.SetProvider(ApiImportsProvider, ApiImportInfo{
+		SharedLibs: sharedLibs,
+		HeaderLibs: headerLibs,
+	})
+}
+
+func GetApiImportSuffix() string {
+	return apiImportNameSuffix
+}