fix(3p/nix): Fix parseDrvPathWithOutputs

At some point the behavior of this function got changed as part of our
cleanup - this fixes it to behave the way the rest of the codebase
expects (and how it is documented in the header) and covers it with a
few tests.

Change-Id: Id4c91232968e73489cd866fb4a2a84bcf20d875e
Reviewed-on: https://cl.tvl.fyi/c/depot/+/1629
Tested-by: BuildkiteCI
Reviewed-by: tazjin <mail@tazj.in>

third_party/nix/src/libstore/derivations.cc

@@ -408,15 +408,15 @@ Hash hashDerivationModulo(Store& store, Derivation drv) {
   return hashString(htSHA256, drv.unparse());
 }
 
-// TODO(tazjin): doc comment?
 DrvPathWithOutputs parseDrvPathWithOutputs(absl::string_view path) {
   auto pos = path.find('!');
   if (pos == absl::string_view::npos) {
     return DrvPathWithOutputs(path, std::set<std::string>());
   }
 
-  return DrvPathWithOutputs(path.substr(pos + 1),
-                            absl::StrSplit(path, absl::ByChar(',')));
+  return DrvPathWithOutputs(
+      path.substr(0, pos),
+      absl::StrSplit(path.substr(pos + 1), absl::ByChar(',')));
 }
 
 Path makeDrvPathWithOutputs(const Path& drvPath,