bp2build: Handle export_generated_header property

The generated_header property resides in BaseCompilerProperties, while
export_generated_header resides in BaseLinkerProperties. Previously
bp2build handled these property structs separately; however, these two
related properties residing in separate structs requires restructuring
the code to allow access to both BaseCompilerProperties and
BaseLinkerProperties for a single axis/configuration combination to
resolve which generated headers are exported/not for the
axis/configuration..

Test: go test soong tests
Test: build/bazel/ci/bp2build.sh
Change-Id: Id150003637fd19d87e8dc5d6941e9f36dc4031dd

bp2build/cc_library_static_conversion_test.go

@@ -997,14 +997,21 @@ genrule {
     cmd: "nothing to see here",
 }
 
+genrule {
+    name: "export_generated_hdr",
+    cmd: "nothing to see here",
+}
+
 cc_library_static {
     name: "foo_static",
     srcs: ["cpp_src.cpp", "as_src.S", "c_src.c"],
-    generated_headers: ["generated_hdr"],
+    generated_headers: ["generated_hdr", "export_generated_hdr"],
+    export_generated_headers: ["export_generated_hdr"],
     include_build_directory: false,
 }`,
 		expectedBazelTargets: []string{`cc_library_static(
     name = "foo_static",
+    hdrs = [":export_generated_hdr"],
     srcs = [
         "cpp_src.cpp",
         ":generated_hdr",