wiggle: witx paths use shell expansion

instead of always being relative to CARGO_MANIFEST_DIR, each use site is
responsible for either putting that variable or another one (set by a
build.rs) at the start of witx paths.

crates/wiggle/generate/Cargo.toml

@@ -20,6 +20,7 @@ proc-macro2 = "1.0"
 heck = "0.3"
 anyhow = "1"
 syn = { version = "1.0", features = ["full"] }
+shellexpand = "2.0"
 
 [badges]
 maintenance = { status = "actively-developed" }