Improve support for tramp source buffers

Ensures load file doesn't use the tramp location of the file for the filename
passed to the nrepl load-file op.  This caused some issues when using the same
repl and source files both locally and over tramp.

Uses an explicit buffer to determine the tramp prefix rather than the 
find-tag-marker-ring.  Changes the signature of cider-jump-to-def-for to add a
buffer argument.