Adds the repository rule xcode_sdk_frameworks (#898)

### What
It scans and configures all SDK frameworks which are required for
explicit module builds.

Specifically, it 1) uses the bazel_tools//tools/osx:xcode_configure.bzl
to fetch all local Xcode infos,
  2) finds all frameworks under the xcode SDK path,
and 3) uses the swiftc -scan-dependencies mode to fetch the dependency
graph among all Swift and Clang SDK modules.

We use swift_module_alias to represent a Swift SDK module and
sdk_clang_module for a Clang SDK module.

This PR is based on the previous attempt to support SDK frameworks:
#562

### Test 

Runs `bazel build --config=explicit_modules --nobuild
@xcode_sdk_frameworks//...` and inspects the generated repository at
`$(bazel info
output_base)/external/_main~xcode_sdk_frameworks~xcode_sdk_frameworks`.


### Next steps:
The dependency among swift_module_alias and sdk_clang_module doesn't
work yet. We need to fix that.

Author: congt

.bazelrc

@@ -65,6 +65,7 @@ build:remote_cache --remote_timeout=3600
 # Configure for explicit module compilation
 build:explicit_modules --features=swift.use_c_modules
 build:explicit_modules --features=swift.emit_c_module
+build:explicit_modules --repo_env=EXPLICIT_MODULES=1
 
 # By default don't upload local results to remote cache, only CI does this.
 build --noremote_upload_local_results

MODULE.bazel

@@ -101,3 +101,14 @@ use_repo(
     apple_non_module_deps,
     "xctestrunner",
 )
+
+# Load xcode_sdk_frameworks
+xcode_sdk_frameworks = use_extension(
+    "//rules:module_extensions.bzl",
+    "xcode_sdk_frameworks",
+    dev_dependency = True,
+)
+use_repo(
+    xcode_sdk_frameworks,
+    "xcode_sdk_frameworks",
+)

rules/explicit_module/sdk_clang_module.bzl

@@ -1,4 +1,5 @@
 load("@build_bazel_rules_swift//swift/internal:feature_names.bzl", "SWIFT_FEATURE_SYSTEM_MODULE")
+load("@build_bazel_rules_swift//swift/internal:providers.bzl", "create_swift_info")
 load("@build_bazel_rules_swift//swift/internal:swift_common.bzl", "swift_common")
 
 def _sdk_clang_module_impl(ctx):
@@ -11,10 +12,16 @@ def _sdk_clang_module_impl(ctx):
         # We need to return CcInfo and its compilation_context. We may also consider to update swift_clang_module_aspect.
         # See https://github.com/bazelbuild/rules_swift/blob/d68b21471e4e9d922b75e2b0621082b8ce017d11/swift/internal/swift_clang_module_aspect.bzl#L548
         CcInfo(compilation_context = cc_common.create_compilation_context()),
+        # Required to add sdk_clang_module targets to the deps of swift_module_alias.
+        # TODO(cshi): create the SwiftInfo correctly
+        create_swift_info(),
     ]
 
 sdk_clang_module = rule(
     attrs = {
+        "deps": attr.label_list(
+            doc = "The deps of the SDK clang module",
+        ),
         "module_map": attr.string(
             doc = """\
 The path to a SDK framework module map.

rules/module_extensions.bzl

@@ -5,6 +5,10 @@ load(
     "rules_ios_dependencies",
     "rules_ios_dev_dependencies",
 )
+load(
+    "//rules:xcode_sdk_frameworks.bzl",
+    "load_xcode_sdk_frameworks",
+)
 load(
     "//tools/toolchains/xcode_configure:xcode_configure.bzl",
     _xcode_configure = "xcode_configure",
@@ -41,3 +45,8 @@ xcode_configure = module_extension(
         ),
     },
 )
+
+def _xcode_sdk_frameworks_impl(_):
+    load_xcode_sdk_frameworks()
+
+xcode_sdk_frameworks = module_extension(implementation = _xcode_sdk_frameworks_impl)