Add the file a symbol is declared in to Reflection (#6613)

* Add the file a symbol is declared in to Reflection

If we move a code-generator to depend on Reflection,
it may need to know which file something was declared in
to properly name generated files.

* Doc comments in reflection, and more precise tests

* Add --project-root flag to flatc, normalize declaraion_file to this root

* fix --project-root stuff

* posixpath

* fix scripts

* format

* rename --project-root to --bfbs-filenames

Also, make it optional, rather than defaulting to `./`, if its not
specified, then don't serialize the filenames.

* bfbs generation

* fix some tests

* uncomment a thing

* add  to project root directory conditionally

* fix

* git clang format

* Added help description and removed != nullptr

* "

* Remove accidental change to docs

* Remove accidental change to docs

* Pool strings

Co-authored-by: Casper Neo <cneo@google.com>

tests/rust_usage_test/outdir/build.rs

@@ -1,6 +1,5 @@
 fn main() {
     use std::process::Command;
-    
     let project_root = std::env::current_dir()
         .unwrap()
         .parent()  // flatbuffers/tests/rust_usage test
@@ -10,7 +9,6 @@ fn main() {
         .parent()  // flatbuffers/
         .unwrap()
         .to_path_buf();
-    
     let sample_schema = {
         let mut s = project_root.to_path_buf();
         s.push("samples");
@@ -35,5 +33,6 @@ fn main() {
         .expect("Failed to generate file");
 
     let genfile = "monster_gen.rs";
-    std::fs::rename(&genfile, out_dir.join("monster_generated.rs")).unwrap();
+    std::fs::rename(&genfile, out_dir.join("monster_generated.rs"))
+        .expect("Could not rename monster_ge.rs to monster_generated.rs");
 }