For dependencies with namespace, the namespace file is always rebuilt

[bobzhang]

This is less critical thanks to the hash of cmi file, but we may need avoid regenerate mlmap fille for bsb -regen

priv>bsb -make-world
IGNORED "dx" in public is ignored since it is notan existing module
[1/3] Building Liba.cmi
ninja: no work to do.
priv>bsb -make-world -- -d explain
IGNORED "dx" in public is ignored since it is notan existing module
ninja explain: loading dyndep file 'src/demo.d'
ninja explain: restat of output Liba.cmi older than most recent input Liba.mlmap (1562232616003517118 vs 1562232632412646892)
ninja explain: Liba.cmi is dirty
ninja explain: loading dyndep file 'src/priv.d'
ninja explain: Liba.cmi is dirty
[1/3] Building Liba.cmi
ninja explain: loading dyndep file 'src/demo.d'
ninja explain: loading dyndep file 'src/list.d'
ninja explain: loading dyndep file 'src/main.d'
ninja: no work to do.