Fab reports contained subroutines as external symbols

While working on Socrates, I've found that code like:
```
program a
...
contains
subroutine b
end subroutine b
end program a
```
reports subroutine `b` as an external symbol (and then finds duplicated symbols, since many subroutine re-implement the same local subroutine :cry:

Reproduce the report with the Fortran program shown in the issue, then trace Fab's symbol-discovery entry point to see why the contained subroutine is classified as external. Done means contained subroutines are excluded from external-symbol reports and duplicate-symbol findings no longer occur for this pattern.

Written by the indexing model from the issue text.