Ignore pre-defined filenames when PIP_REQUIREMENTS_FILE is set

What does this MR do?

This fixes a bug where gemnasium-python scans a dependency file other than the one specified in PIP_REQUIREMENTS_FILE.

• When PIP_REQUIREMENTS_FILE is set,
  • Use it as the only supported filename for pip.
  • Ignore dependency files handled by package managers other than pip.
  • Show a warning if has a directory component.

What are the relevant issue numbers?

PIP_REQUIREMENTS_FILE variable is ignored by py... (gitlab-org/gitlab#350949 - closed)

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated for GitLab EE, if necessary
• Documentation created/updated for this project, if necessary
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Job definition updated, if necessary
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• Conforms to the code review guidelines
• Conforms to the Go guidelines
• Security reports checked/validated by reviewer