`dbmate` package was starting to get a bit polluted. This PR migrates each driver into a separate package, with clean separation between each.
In addition:
* Drivers are now initialized with a URL, avoiding the need to pass `*url.URL` to every method
* Sqlite supports a cleaner syntax for relative paths
* Driver tests now load their test URL from environment variables
Public API of `dbmate` package has not changed (no changes to `main` package).
Prior to this commit, we released two linux binaries:
* `dbmate-linux-amd64` (built with cgo, dynamically linked)
* `dbmate-linux-musl-amd64` (built without cgo, statically linked, no sqlite support)
The statically linked binary is desirable for alpine linux users (or anyone else using musl libc or minimal docker images). The original reason for having two separate binaries was that the easiest method to create a static binary for go is to set `CGO_ENABLED=0`, but unfortunately this also prevented us from building sqlite (which requires cgo).
With this commit, all linux and windows binaries are explicitly statically linked while leaving cgo enabled. Hat tip to https://www.arp242.net/static-go.html which explained the necessary flags to enable this.
As an added bonus, the `dbmate` docker image now now uses a `scratch` base rather than `gcr.io/distroless/base`, reducing the image size from 26.7 MB to 9.8 MB.
Building with `CGO_ENABLED=0` results in a binary which can run on Alpine linux (with musl libc), but it also disables SQLite support.
Adding an additional target allows people to decide this tradeoff for themselves.
