[Feature] Support referencing functions via virtual path

[DarkWanderer]

Is your feature request related to a problem? Please describe.
I would like to run tests in a project which has CBA-like structure - multiple subdirectories and cross-addin function calls. To do that, I need to be able to execute function from a PBO -

See also: #186 #195

Describe the solution you'd like
Allow --input-sqf parameter to utilize virtual path when loading file, e.g. sqfvm --input-pbo my.pbo --input-sqf \my\pbo\virtual\path.sqf