fix: make path resolution relative to project root

this was already how it mostly behaved but there seemed to be exceptions
like LSP warnings (in these tests themselves)

const-macros-proc/src/lib.rs

@@ -1,5 +1,5 @@
 #![feature(proc_macro_value)]
-use std::{fs::File, io::Read};
+use std::{fs::File, io::Read, path::Path};
 
 use proc_macro::{Delimiter, Group, Literal, Punct, TokenStream, TokenTree};
 use sha2::Digest;
@@ -114,7 +114,10 @@ fn get_sha256sum(buf: &[u8]) -> [u8; 32] {
 pub fn include_asset(input: TokenStream) -> TokenStream {
     let path = read_path_literal(input);
     let mut buf = Vec::new();
-    File::open(path)
+
+    let manifest_dir = std::env::var("CARGO_MANIFEST_DIR").expect("CARGO_MANIFEST_DIR to be set");
+
+    File::open(Path::new(&manifest_dir).join(&path))
         .expect("couldn't open file")
         .read_to_end(&mut buf)
         .expect("error reading file");