internal/buildinfo: do module-level analysis with no PCLN table

No PCLN table means we just have buildinfo, which is exactly the same
scenario as when the binary is stripped. We should hence support it.

Fixes golang/go#59731

Change-Id: Ib6dd072f4827bc98bbbe5e686c81b5d72246a58b
Reviewed-on: https://go-review.googlesource.com/c/vuln/+/560375
Reviewed-by: Maceo Thompson <maceothompson@google.com>
Run-TryBot: Zvonimir Pavlinovic <zpavlinovic@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Roland Shoemaker <roland@golang.org>

internal/buildinfo/additions_scan.go

@@ -77,10 +77,9 @@ func ExtractPackagesAndSymbols(bin io.ReaderAt) ([]*packages.Module, []Symbol, *
 
 	pclntab, textOffset := x.PCLNTab()
 	if pclntab == nil {
-		// TODO(https://go.dev/issue/59731): if we have build information, but
-		// not PCLN table, we should be able to fall back to much higher
-		// granularity vulnerability checking.
-		return nil, nil, nil, errors.New("unable to load the PCLN table")
+		// If we have build information, but not PCLN table, fall
+		// back to much higher granularity vulnerability checking.
+		return debugModulesToPackagesModules(bi.Deps), nil, bi, nil
 	}
 	lineTab := gosym.NewLineTable(pclntab, textOffset)
 	if lineTab == nil {