chore: Ensure collector build funcs are private

Collector builder functions are only used internally in the `collector`
package, and shouldn't needlessly be exposed as part of the package API
to downstream clients.

Signed-off-by: Ben Reedy <breed808@breed808.com>

collector/system_test.go

@@ -5,5 +5,5 @@ import (
 )
 
 func BenchmarkSystemCollector(b *testing.B) {
-	benchmarkCollector(b, "system", NewSystemCollector)
+	benchmarkCollector(b, "system", newSystemCollector)
 }