Speeding up DCD in big projects

[WebFreak001]

So it turns out the `resolveImportLocation` function, which is 
called on the first semantic pass, currently on Windows takes 
like 50ms to complete once for just about 60 import paths. In a 
big project like serve-d (as source code project) this would sum 
up to over 7 minutes. (in release mode just around 2 minutes)

I optimized the import path check code and now in DCD debug mode 
you might get a 83x speed up for your first completion and in DCD 
release mode you might get a 26x speed up for your first 
completion. In serve-d (as source code project) this translated 
to the first completion now only taking 5 seconds.

I think in big projects optimizing the startup time too is pretty 
essential and this was a very easy improvement I immediately 
found using the debugger. The example times are made using DCD 
~master with my dsymbol PR on and off inside the source code of 
serve-d trying to auto complete extension.d:860

This was a very easy to find and improve spot, there might be 
more low hanging fruit to get the first parse and completion even 
quicker than 5s.

The PR: https://github.com/dlang-community/dsymbol/pull/151