Sources have to be fetched online at some point anyways. If Internet goes out after you've fetched the sources, then you can still build the packages.
Regarding user packages: it is not a good idea to use them in PKGBUILDs for several reasons, first one being that the user is free to change the source code of Go package locally (change branch, change code). The PKGBUILD must be reproduicible, so it's better to start afresh. -- Pierre Neidhardt
