Migrate `Finder` component to `OsPath`, fixed #24616

For each module in a GHCi session, we keep alive one `ModLocation`.
A `ModLocation` is fairly inefficiently packed, as `String`s are
expensive in memory usage.

While benchmarking the agda codebase, we concluded that we keep alive
around 11MB of `FilePath`'s, solely retained by `ModLocation`.

We provide a more densely packed encoding of `ModLocation`, by moving
from `FilePath` to `OsPath`. Further, we migrate the full `Finder`
component to `OsPath` to avoid unnecessary transformations.
As the `Finder` component is well-encapsulated, this requires only a
minimal amount of changes in other modules.

We introduce pattern synonym for 'ModLocation' which maintains backwards
compatibility and avoids breaking consumers of 'ModLocation'.