query macros with file is broken with cargo workspace

[AtkinsChang]

Regression related to v0.5.6

when sqlx is used in sub-project, the macro use with SQL files behave differently with previous version: the file must exist in both root project and sub-project to compile.

this regression is introduced by #1332,
the path is different in following path:
https://github.com/launchbadge/sqlx/blob/master/sqlx-macros/src/query/input.rs#L144
https://github.com/launchbadge/sqlx/blob/master/sqlx-macros/src/query/input.rs#L123-L125

let me know if you need the reproduce scenario and fix
I'll do it when I have time

[abonander]

If you feel like fixing this, that'd be great, otherwise I'll try to get to this when I can and kick out another patch release.