Scope of every dependency is install in setup.cfg

### Description
Scope of every dependency is install in setup.cfg

### How To Reproduce
Parse this cfg file with SetupCfgHandler.
```
[metadata]
name = resolvelib
version = attr: resolvelib.__version__
description = Resolve abstract dependencies into concrete ones
url = https://github.com/sarugaku/resolvelib
author = Tzu-ping Chung
author_email = uranusjr@gmail.com
long_description = file: README.rst
license = ISC License
keywords =
dependency
resolution
classifier =
Development Status :: 3 - Alpha
Intended Audience :: Developers
License :: OSI Approved :: ISC License (ISCL)
Operating System :: OS Independent
Programming Language :: Python :: 2
Programming Language :: Python :: 3
Topic :: Software Development :: Libraries :: Python Modules

[options]
package_dir =
= src
packages = find:
include_package_data = true
zip_safe = false

[options.packages.find]
where = src

[options.extras_require]
examples =
html5lib
packaging
pygraphviz
requests
lint =
black
flake8
mypy
isort
types-requests
test =
commentjson
packaging
pytest
release =
build
towncrier
twine

[bdist_wheel]
universal = 1

[flake8]
max-line-length = 88
select = C,E,F,W,B
ignore = E203, W503
exclude =
.git,
.venv,
.venvs,
.nox,
__pycache__,
build,
dist,
*.pyi

[egg_info]
tag_build =
tag_date = 0
```
Scope of every dependency is install, but it should be different

Expectation: DependentPackage(purl='pkg:pypi/html5lib', extracted_requirement='html5lib', scope='examples')
Computed: DependentPackage(purl='pkg:pypi/html5lib', extracted_requirement='html5lib', scope='install')

* What OS are you running on? Linux
* What version of scancode-toolkit was used to generate the scan file
* What installation method was used to install/run scancode? source download