client: allows fsCache to be publicly accessable for go audit.

fsCache is the only cache implementation available. In order to be
integrated in goaudit, it needs to be made publicly accessable as
go audit and cache do not live in the same repo. fsCache will be made
private again once go audit and client live in the same space in the
near future.

Change-Id: I4dd86f407ce83f2162e8a1921f86643bbefdd456
Reviewed-on: https://team-review.git.corp.google.com/c/golang/vulndb/+/1033548
Reviewed-by: Roland Shoemaker <bracewell@google.com>

client/cache.go

@@ -43,8 +43,14 @@ type Cache interface {
 
 type fsCache struct{}
 
+// NewFsCache returns a fresh filesystem cache.
+// TODO: remove once the cache implementation reaches the go tooling repo.
+func NewFsCache() Cache {
+	return &fsCache{}
+}
+
 // should be cfg.GOMODCACHE when doing this inside the cmd/go/internal
-var cacheRoot = filepath.Join(build.Default.GOPATH, "/pkg/mod/cache/downlaod/vulndb")
+var cacheRoot = filepath.Join(build.Default.GOPATH, "/pkg/mod/cache/download/vulndb")
 
 type cachedIndex struct {
 	Retrieved time.Time