Could not use custom namespace on Boost 1.89.0: Unable to find file or target named boost_align

Hello!

I am opening this issue as a reference to https://github.com/conan-io/conan-center-index/issues/28978

An error was reported when trying to use Boost custom namespace, but of course, it could be a bug in the Conan recipe.

Still, I tried to build from scratch without Conan, and I reached the following steps:

```bash
#!/bin/bash

BOOST_VERSION=1_89_0
BOOST_BASE_VERSION=$(echo ${BOOST_VERSION} | tr _ .)
MYBOOST_DIR=/tmp/boost-${BOOST_BASE_VERSION}/myboost
INSTALL_DIR=/tmp/boost-${BOOST_BASE_VERSION}/install

set -ex

wget https://archives.boost.io/release/${BOOST_BASE_VERSION}/source/boost_${BOOST_VERSION}.tar.bz2
tar -xjf boost_${BOOST_VERSION}.tar.bz2
cd boost_${BOOST_VERSION}

./bootstrap.sh --with-toolset=gcc
./b2 tools/bcp

rm -rf ${MYBOOST_DIR}
mkdir -p ${MYBOOST_DIR}
./dist/bin/bcp --boost=. --namespace=myboost \
align atomic charconv chrono container context contract coroutine \
date_time exception fiber filesystem graph iostreams json \
locale log math nowide process program_options random regex \
serialization stacktrace test thread timer type_erasure url wave \
config build boost_install headers \
${MYBOOST_DIR}

cd ${MYBOOST_DIR}
./bootstrap.sh --with-toolset=gcc

./b2 \
toolset=gcc \
variant=release \
link=static \
threading=multi \
cxxstd=17 \
--prefix=${INSTALL_DIR} \
install
```

However, in the final steps, when running b2 in the new subset folder, I have:

```
error: Unable to find file or target named
error: '/boost/align//boost_align'
error: referred to from project at
error: '/tmp/boost-1.89.0/myboost/libs/atomic/build'
```

You can read my full log here: [boost-1.89.0-build-namespace.log](https://github.com/user-attachments/files/23789244/boost-1.89.0-build-namespace.log)

It seems to be related to the modular Boost, causing a missing dependency. But as I'm not sure about it, I opened this issue first to get some support on this case.

#### Environment

Boost version: 1.89.0
OS: GNU/Linux 6.8.0
Distro: Ubuntu 24.04
Compiler: gcc (Ubuntu 11.4.0-1ubuntu1~22.04.2) 11.4.0

Please, let me know if you need extra information that can help to clarify the case. Regards!