Refactor drivers into separate packages (#179)

`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).

pkg/dbutil/dbutil_test.go

@@ -0,0 +1,58 @@
+package dbutil_test
+
+import (
+	"database/sql"
+	"testing"
+
+	"github.com/amacneil/dbmate/pkg/dbutil"
+
+	_ "github.com/mattn/go-sqlite3" // database/sql driver
+	"github.com/stretchr/testify/require"
+)
+
+func TestDatabaseName(t *testing.T) {
+	t.Run("valid", func(t *testing.T) {
+		u := dbutil.MustParseURL("foo://host/dbname?query")
+		name := dbutil.DatabaseName(u)
+		require.Equal(t, "dbname", name)
+	})
+
+	t.Run("empty", func(t *testing.T) {
+		u := dbutil.MustParseURL("foo://host")
+		name := dbutil.DatabaseName(u)
+		require.Equal(t, "", name)
+	})
+}
+
+func TestTrimLeadingSQLComments(t *testing.T) {
+	in := "--\n" +
+		"-- foo\n\n" +
+		"-- bar\n\n" +
+		"real stuff\n" +
+		"-- end\n"
+	out, err := dbutil.TrimLeadingSQLComments([]byte(in))
+	require.NoError(t, err)
+	require.Equal(t, "real stuff\n-- end\n", string(out))
+}
+
+// connect to in-memory sqlite database for testing
+const sqliteMemoryDB = "file:dbutil.sqlite3?mode=memory&cache=shared"
+
+func TestQueryColumn(t *testing.T) {
+	db, err := sql.Open("sqlite3", sqliteMemoryDB)
+	require.NoError(t, err)
+
+	val, err := dbutil.QueryColumn(db, "select 'foo_' || val from (select ? as val union select ?)",
+		"hi", "there")
+	require.NoError(t, err)
+	require.Equal(t, []string{"foo_hi", "foo_there"}, val)
+}
+
+func TestQueryValue(t *testing.T) {
+	db, err := sql.Open("sqlite3", sqliteMemoryDB)
+	require.NoError(t, err)
+
+	val, err := dbutil.QueryValue(db, "select $1 + $2", "5", 2)
+	require.NoError(t, err)
+	require.Equal(t, "7", val)
+}