Move ParseDSN into sub package

### Issue description

Would be good to have ParseDSN/Config struct in another package than mysql since the init function registers the mysql driver, so you can't access it without it registering.

My use case is I wanted to have a simple port check for mysql, obtaining the `Addr` from ParseDSN. Problem is that since it registers the driver, if I subsequently import my utility package it's not easy to see that it's already registered that driver and the app will panic.
### Error log

```
sql: Register called twice for driver mysql

```