Prevents loss of config modifications done by NuGet packages on install/update/uninstall, when you have a config transform setup where the root config file (e.g. \web.config) is a result of transformation from a source config file (e.g. \Configurations\web.config).
This is done by merging any and all config files in each directory one level below the solution file back to the corresponding source config file, as defined in the directory's .wpp.targets file (the first if multiple are found).
Note that the source config file will also get the results of the transformations in the .debug.config file (or whichever configuration was last built), since the transformed config file will have those transformations in it as well as any modifications done by NuGet packages - so you must carefully which changes should be kept and which should be discarded in the source config file. TIP: Add an empty configuration ("Base") in Configuration Manager and build in that configuration to ensure the source and transformed config files are equal, thereby avoiding unwanted merging of local/debug values into the source config.