Add bp2build support for cc_library_static.

Also refactor bp2build-related code for cc_library_headers.

(Retry of previous CL after the presubmit failed to detect a merge conflict)

Test: Added unit test.

Test: bp2build-sync.py write; bazel build //bionic/... still works (but bp2build is disabled for most cc_library_static targets for other reasons)
Change-Id: I2f4405c2fea305623bbc6daaaf62808b0c074216

bp2build/cc_library_headers_conversion_test.go

@@ -123,7 +123,8 @@ cc_library_headers {
     name: "foo_headers",
     export_include_dirs: ["dir-1", "dir-2"],
     header_libs: ["lib-1", "lib-2"],
-    export_header_lib_headers: ["lib-1", "lib-2"],
+
+    // TODO: Also support export_header_lib_headers
     bazel_module: { bp2build_available: true },
 }`,
 			expectedBazelTargets: []string{`cc_library_headers(