Fixing false positive missing relation warnings.

Instances can be created without generic maps or port maps, in the form LABEL : entity. The existing regex captures these, but also finds false positives in component, record, and function definition sections, such as generic_name : natrual. new_dep_solver then compains that work.natural is not satisfied by any source file.
This commit creates regex patterns for component, function, and record definitions to remove them from the input buffer strings. The buf variable is reassigned after each re.sub call, to affect the removal.
Replacement strings are added to the buf, when a regex has matched - to help with debugging.
Finally, re.subn() replaced with re.sub() for simplicity.