internal/worker: create synthetic modules in compare mode

This makes compare mode consistent with other modes and it will reduce
the number of errors generated and logged.

Change-Id: I6e57f12fe4453990ff1abf6209687a7d03bb574b
Reviewed-on: https://go-review.googlesource.com/c/pkgsite-metrics/+/524576
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Zvonimir Pavlinovic <zpavlinovic@google.com>
Reviewed-by: Maceo Thompson <maceothompson@google.com>

internal/worker/govulncheck_scan.go

diff --git a/internal/worker/govulncheck_scan.go b/internal/worker/govulncheck_scan.go
index f61cf04..b3cb27d 100644
--- a/internal/worker/govulncheck_scan.go
+++ b/internal/worker/govulncheck_scan.go
@@ -217,7 +217,7 @@
 	err = doScan(ctx, baseRow.ModulePath, info.Version, s.insecure, func() (err error) {
 		inputPath := moduleDir(baseRow.ModulePath, info.Version)
 		defer derrors.Cleanup(&err, func() error { return os.RemoveAll(inputPath) })
-		const init = false
+		const init = true
 		if err := prepareModule(ctx, baseRow.ModulePath, info.Version, inputPath, s.proxyClient, s.insecure, init); err != nil {
 			log.Errorf(ctx, err, "error trying to prepare module %s", baseRow.ModulePath)
 			return nil