cmd/toolexec: rewrite of packages that don't import errtrace

[prashantv]

As discussed in #17 (comment), toolexec can't be used to add new imports to a package.

However, we can workaround this using go:linkname which requires unsafe, and doesn't work for generic functions.

We can support this in an unsafe mode, probably using an allow list to determine what packages to rewrite.

Since go:linkName can't be used with generic functions, we can't use Wrap2 etc, we can try rewriting those to
use a normal Wrap call as suggested.

[nyobe]

Using -overlay seems to work quite well, sort of sidestepping the problem of toolexec being called too late:

# ask the errtrace tool which files is would update
# write the updated files to a tempdir
# create an overlay file to tell go to substitute the updated files for the local files
.make/errtrace_overlay.json: $(shell go tool errtrace -l ./...)
	@OUT="$$(mktemp -d -t errtrace.XXXXXX)"; \
	FILES="$^"; \

	for f in $$FILES; do \
		mkdir -p "$$OUT/$$(dirname $$f)"; \
		go tool errtrace "$$f" > "$$OUT/$$f"; \
	done;

	echo $$FILES | \
    	jq -R -s --arg out "$$OUT" \
    	'{ "Replace": ( split(" ") | map(select(length > 0) | { (.): ($$out + "/" + .) }) | add ) }' \
    	> $@


# now you can use the overlay file in tests
go test -overlay=.make/errtrace_overlay.json

This is slow right now due to needing to invoke go tool for each individual file, but maybe errtrace could learn to generate an overlay file itself

[prashantv]

The overlay option seems much cleaner than the unsafe go:linkname approach, but it does place a higher burden on the caller, compared to adding the-toolexec flag at build-time.

In #128, I've added support for using go:linkname to allow other packages to use errtrace.Wrap even if they haven't imported the package. In my local testing, it's worked well, though it requires at least one package in the final binary to have imported errtrace.

In practice, I imagine -overlay would likely have a similar need, since users would need errtrace in their go.mod. We'll see how well the go:linkname hack works, and reconsider -overlay if required.