internal/govulncheck: add WantPackages scan level

Adds a check for if govulncheck is at a scan level more precise than
module level scanning (can also be used to check if the scan is at a
module level).

Change-Id: I5231e6a88c865a978a3f570d558705754e60501a
Reviewed-on: https://go-review.googlesource.com/c/vuln/+/522737
Run-TryBot: Maceo Thompson <maceothompson@google.com>
Reviewed-by: Ian Cottrell <iancottrell@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

internal/govulncheck/govulncheck.go

@@ -156,3 +156,7 @@ const (
 // WantSymbols can be used to check whether the scan level is one that is able
 // to generate symbols called findings.
 func (l ScanLevel) WantSymbols() bool { return l == scanLevelSymbol }
+
+// WantPackages can be used to check whether the scan level is one that is able
+// to generate package
+func (l ScanLevel) WantPackages() bool { return l == scanLevelPackage || l == scanLevelSymbol }