Adding ccache-like output caching to dmd

[Per Nordlöw]

On Tuesday, 29 December 2020 at 12:49:45 UTC, Stefan Koch wrote:
> The issue is that because of string imports you don't know the 
> full set of files you are depending on.
> which means any change can cause any file to be required.

If we, in dmd, during the initial (uncached) build log all the 
imported files including string imports and output them to a 
cache description together with their individual content hashes 
and pessimistically rebuild every time anything changes I don't 
see how this can be an issue. Can you elaborate on which case 
I've missed?