resource: Fix golint issues

resource: Fix golint issues

Fix godoc comments and the following:

resource/image.go:33:2: a blank import should be only in a main or test package, or have a comment justifying it
resource/image.go:35:2: a blank import should be only in a main or test package, or have a comment justifying it
resource/image.go:41:2: a blank import should be only in a main or test package, or have a comment justifying it
resource/image.go:538:10: if block ends with a return statement, so drop this else and outdent its block
resource/resource.go:54:7: exported const DefaultResourceType should have comment or be unexported
resource/resource.go:233:1: receiver name r1 should be consistent with previous receiver name r for Resources
resource/resource.go:254:1: receiver name r1 should be consistent with previous receiver name r for Resources