Hold a cgo file's directory whole: its preamble may include from below it (hq ADR 0267, review)

internal/builder/gosource.go

@@ -351,6 +351,8 @@ func goFileReads(file string) (imports, embeds []string, err error) {
 					}
 				}
 			}
+			// A cgo file may include from below its directory: the directory is held whole.
+			embeds = append(embeds, ".")
 			continue
 		}
 		imports = append(imports, ip)

internal/builder/gosource_test.go

@@ -304,3 +304,15 @@ func TestNativeSourcesHoldTheirDirectoryWhole(t *testing.T) {
 		}
 	}
 }
+
+// A cgo file may include from below its directory with nothing native beside it: its directory is held whole.
+func TestACgoFileHoldsItsDirectoryWhole(t *testing.T) {
+	got, err := GoBuildSource(aGoTree(t, map[string]string{"go.mod": "module x\n",
+		"p/main.go": "package main\n\n// #include \"inc/x.h\"\nimport \"C\"\n", "p/inc/x.h": ""}), "p")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !SourceHolds(got, "p/inc/x.h") {
+		t.Fatalf("a header a cgo preamble includes is not held: %v", got)
+	}
+}