Not all source files get extracted with bcp

### Version of Boost: 1.76
When extracting `Boost.JSON` from the `boost` sources using the `bcp` tool, it seems that not all files get properly copied.

### Steps necessary to reproduce the problem:
```
curl --silent --show-error -L https://boostorg.jfrog.io/artifactory/main/release/1.76.0/source/boost_1_76_0.tar.bz2 -o ./boost.tar.bz2
mkdir boost_build
tar xf boost.tar.bz2 -C ./boost_build --strip-components=1
cd boost_build && ./bootstrap.sh && ./b2 tools
mkdir ~/boost_json_extract/
./dist/bin/bcp boost/json.hpp ~/boost_json_extract/
tree ~/boost_json_extract > content_stage1.txt
./dist/bin/bcp boost/json/src.hpp ~/boost_json_extract/
tree ~/boost_json_extract > content_stage2.txt
```

The following are the content files generated.
[content_stage1.txt](https://github.com/boostorg/json/files/7531724/content_stage1.txt)
[content_stage2.txt](https://github.com/boostorg/json/files/7531725/content_stage2.txt)

Please note that the `content_stage1.txt` file does not contain the `src.hpp` file - it gets copied only when passing it to `bcp` explicitly (so exists in the `content_stage2.txt` file), however none of these text files contains the `src.cpp` file entry.

### All relevant compiler information
Apple clang version 13.0.0 (clang-1300.0.29.3), MacBook Pro (13-inch, M1, 2020).
It seems to be compiler independent.