fix: handle case of missing .cmj when trying to inline external field

[anmonteiro]

this is the melange part of the fix for #658.

• Melange looks for some inlining information in the .cmj files for implementations. When compiling virtual libraries, .cmj files are deferred until a specific implementation has been selected, so they won't be available at this time
• There's a more thorough explanation in the code about what the fix does.

fixes #658

TODO:

• add a blackbox-test