dmd: Encourage more inlining across modules

[Andrew Pennebaker]

I read from the FreeBSD documentation that dmd can compile many 
source trees without having to explicitly list all the filenames, 
however a lot of inlining optimizations are missed.

https://dlang.org/dmd-freebsd.html

Curious, could dmd separate out the module identification logic 
into an earlier phase, so that it can perform the same 
optimization regardless of whether users remember to explicitly 
name their files?