Bp2build: handle embedded structs as blueprint

For structs that are embedded, Blueprint does not nest under the
embedded name, flattening them into the original struct for blueprint
files (e.g.
9fd2ed93df:proptools/unpack_test.go;l=402-431;drc=3adb2409648d6f8b25354ac47f083dae87731f10).
We should do the same for bp2build.

This will also allow us to embed structs for bp2build conversion
allowing more reuse.

Test: go test bp2build tests
Change-Id: I9ce088462adaf59bffa80bea76cd488e31f98e9d

bp2build/build_conversion_test.go

@@ -319,6 +319,30 @@ custom {
         "//build/bazel/platforms/arch:x86": [":dep"],
         "//conditions:default": [],
     }),
+)`,
+			},
+		},
+		{
+			blueprint: `custom {
+    name: "embedded_props",
+    embedded_prop: "abc",
+    bazel_module: { bp2build_available: true },
+}`,
+			expectedBazelTargets: []string{`custom(
+    name = "embedded_props",
+    embedded_attr = "abc",
+)`,
+			},
+		},
+		{
+			blueprint: `custom {
+    name: "ptr_to_embedded_props",
+    other_embedded_prop: "abc",
+    bazel_module: { bp2build_available: true },
+}`,
+			expectedBazelTargets: []string{`custom(
+    name = "ptr_to_embedded_props",
+    other_embedded_attr = "abc",
 )`,
 			},
 		},