We use 'go list -json -export' to locate required modules. This works
fine to locate direct module dependencies; since we're building in the
current module, we run 'go list' in the correct directory.
However, if we're building one of those module dependencies, and it has
other module dependencies of its own, we would fail with cryptic errors
like:
typecheck error: [...] go list error: updates to go.sum needed, disabled by -mod=readonly
This is because we would try to run 'go list' outside of the main
module, probably inside the module cache. Instead, use a $GARBLE_DIR env
var from the top-level 'garble build' call to always run 'go list' in
the original directory.
We add a few small modules to properly test this.
Updates #9.
Daniel Martí