Correctly substitute from different storeDir

Originally, the test was only checking for different “real” storeDir.
That’s an easy case to handle, but the much harder one is if different
virtual store dirs are used. To do this, we need the SubstitutionGoal
to know about the ca, so it can recalculate the path to copy it over.
An important note here is that the store path passed to copyStorePath
needs to be one for srcStore - so that queryPathInfo works properly.

This also adds an error message when the store path from queryPathInfo
is different from the one we requested.

src/libstore/store-api.hh

@@ -855,4 +855,6 @@ std::string makeFixedOutputCA(FileIngestionMethod method, const Hash & hash);
 /* Split URI into protocol+hierarchy part and its parameter set. */
 std::pair<std::string, Store::Params> splitUriAndParams(const std::string & uri);
 
+std::optional<std::string> getDerivationCA(const BasicDerivation & drv);
+
 }