Run coverage over all packages beneath COVERAGE_MOD_DIR (#573)

This avoids any issue with tests that live outside the package in
which the code they're testing resides, e.g.
api/trace/b3_propagator.go

Makefile

@@ -19,7 +19,7 @@ endif
 
 GOTEST_MIN = go test -v -timeout 30s
 GOTEST = $(GOTEST_MIN) -race
-GOTEST_WITH_COVERAGE = $(GOTEST) -coverprofile=coverage.txt -covermode=atomic
+GOTEST_WITH_COVERAGE = $(GOTEST) -coverprofile=coverage.txt -covermode=atomic -coverpkg=./...
 
 .DEFAULT_GOAL := precommit