Fixing issue #373 related to modified-files argument

[SimoneScalco]

Fix for issue #373 related to the usage of modified files.

The usage of modified-files generated an AttributeError when creating an advisory record in build_advisory_record(...).
This easy fix changes the datamodel/advisory.py file. More specifically, it was necessary to change this part of the code:

...
if modified_files and len(modified_files) > 0:
    advisory_record.files.update(set(modified_files.split(",")))
...

Since modified_files is already a set at that point of the code, I changed it to:

...
if modified_files and len(modified_files) > 0:
    advisory_record.files.update(modified_files)
...

I checked other parts of the source code in which modified_files is used. Consequently, I also changed the type annotation related to modified_files in build_advisory_record(...).
As an example, I tested again several different samples. For instance, for CVE-2021-40690 (the same sample posted in the issue) I tested again the following command:

python cli/main.py CVE-2021-40690 --repository https://github.com/apache/santuario-xml-security-java --use-nvd --version-interval 2.2.2:2.2.3 --modified-files KeyInfo,KeyReferenceInfo,RetrievalMethod --use-backend=never --filter-extensions=java

After the fix Prospector correctly processes the modified-files argument and provides a report (the report includes all the files that have been specified manually):

Initialization   [OK]
Processing advisory   [OK]
Git repository cloning   [OK]
  Fixing commit found in the advisory references

Candidate filtering
   [OK]

Processing commits: 100%|███████████████████████████████████████████████████████████████████████████████████████████████| 12/12 [00:03<00:00,  3.07commit/s]
  [OK]
Candidate analysis   [OK]
[OK]
Generating report
   Report saved in prospector-report   [OK]
  Execution time: 19.010s

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