Move the getBuildLog implementation to its own implementation file

Keep the header minimal and clean

src/libstore/log-store.cc

@@ -0,0 +1,12 @@
+#include "log-store.hh"
+
+namespace nix {
+
+std::optional<std::string> LogStore::getBuildLog(const StorePath & path) {
+    auto maybePath = getBuildDerivationPath(path);
+    if (!maybePath)
+        return std::nullopt;
+    return getBuildLogExact(maybePath.value());
+}
+
+}