The wormhole sdk is a new go module in the sdk/ directory. This
initially contains the *_consts.go files from the common package in the
top-level sdk package and the entire vaa package as a sub-package.
For go reasons this needs to be in the sdk directory itself (rather than
a sdk/go subdir). To prevent the go tooling from looking into the other
non-go subdirs, add an empty go.mod file in each one. See
golang issue 42965 for more details on why we can't have nice
things (I'm deliberately not linking to stop github from spamming that
issue).
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