Fix absolute migration directory support

[amacneil]

Closes #403

[nielslanting]

I think this commit broke Windows paths when working with the Go embed package. I'm trying to use dbmate as a library as specified in https://github.com/amacneil/dbmate#use-dbmate-as-a-library.

readMigrationsDir is called with parameter dir ./db/migrations.
readMigrationsDir uses filepath.Clean.
I assume on Linux machines this will turn it into: db/migrations while on Windows this will get turned into db\\migrations.

https://pkg.go.dev/path/filepath#Clean has different behaviour on Windows.

On Windows, Clean does not modify the volume name other than to replace occurrences of "/" with \. For example, Clean("//host/share/../x") returns \\host\share\x.

db\\migrations is not a valid path for the embedded fs. So it's unable to read the files/dir from the embedded fs.