Support bundling DLLs from $OUT_DIR

I'm building a binary distributable that relies on a library crate, which itself dynamically links to SDL2. For windows builds, the build.rs of that library sticks the correct x86_64 or i686 DLLs in the $OUT_DIR location for cargo so they will be present alongside the binary.

Currently the `include` feature would work for this, but I have no way of naming $OUT_DIR in Cargo.toml since it's not set until build time and globs are not supported. Also the specific list of files may change depending on the target platform.

I did see #194 but this request is a bit more limited in scope as these are build artifacts, not a generic random path outside $CARGO_MANIFEST_DIR and the dynamic file list makes this feel like a different feature request.

Start by examining the existing `include` feature and how Cargo.toml paths are resolved, then review the library crate's `build.rs` output in `$OUT_DIR`. Done means cargo-dist can bundle the target-specific DLLs emitted there, including a dynamically determined file list, alongside the distributable binary.

Written by the indexing model from the issue text.