Conditionally apply rustdoc flags to third party crates

This CL changes the logic in rust/builder.go so that some rustdoc flags
are only applied to external crates.  This will allow us to since
warnings and deal with soft-failures in external crates while allowing
us to be more strict with our internal Rust code.

Bug: 195136952
Test: m rustdoc
Change-Id: Icdde304bbbb323cae9657e8f842f58ae79e811ce

cc/compiler_test.go

@@ -16,6 +16,8 @@ package cc
 
 import (
 	"testing"
+
+	"android/soong/android"
 )
 
 func TestIsThirdParty(t *testing.T) {
@@ -32,12 +34,12 @@ func TestIsThirdParty(t *testing.T) {
 		"bionic/libc",
 	}
 	for _, path := range thirdPartyPaths {
-		if !isThirdParty(path) {
+		if !android.IsThirdPartyPath(path) {
 			t.Errorf("Expected %s to be considered third party", path)
 		}
 	}
 	for _, path := range nonThirdPartyPaths {
-		if isThirdParty(path) {
+		if android.IsThirdPartyPath(path) {
 			t.Errorf("Expected %s to *not* be considered third party", path)
 		}
 	}