Problems with `infer_schema` and relative paths, when running `cargo build` in non-root dir.

[dpc]

When specifying the db as a relative path (sqlite), infer_schema macro will use a different location, depending on which directory the cargo build is started in.

Expected behavior

Typically cargo build works correctly no matter where invoked, so I would expect infra_schem to use a relative directory against the root of the project, and not CWD where cargo build was invoked. Because it's not the case eg. building in Neomake fails. (This is a followup of #1142 .)

Steps to reproduce

> cat .env 
DATABASE_URL=test.db
> cat src/schema.rs 
infer_schema!("dotenv:DATABASE_URL");
> cargo build
   Compiling foo v0.1.0 (file:///home/dpc/lab/rust/foo)
    Finished dev [unoptimized + debuginfo] target(s) in 3.74 secs
> cd src; touch bin.rs; cargo build
   Compiling foo v0.1.0 (file:///home/dpc/lab/rust/foo)
error[E0432]: unresolved import `self::schema::reviews`
  --> main.rs:25:19
   |
25 | use self::schema::reviews::dsl::*;
   |                   ^^^^^^^ Could not find `reviews` in `schema`

[...]

[sgrif]

It seems like this is more of a problem of using a relative path for your database URL, rather than a bug in Diesel. We don't really have any behavior here other than passing what you give us unmodified to SQLite.