o.c.opibuilder: generalise check whether file exists

Allows 'workspace-absolute' paths as well as 'filesystem-absolute'
paths.  See cs-studio issue 648.
o.c.opibuilder: use existing function to check for OPI existence

This is still being debated in #657.
Merge pull request #657 from dls-controls/bug648v2

o.c.opibuilder: generalise check whether file exists