Parse pip flags from requirements_lock.txt #438

Description

As a follow up to #432 and the issue brought up by @skliarpawlo we aren't capturing pip flags that may be passed in the requirements_lock.txt file.

E.g
requirements_lock.txt:

--index-url=some.index.com/simple
foo > 0.0.0
bar==1.0.0
...

--index-url is ignored, but in pip_install it works because the requirements file is passed verbatim to pip there.

Describe the solution you'd like

Let's use pip to parse those arguments out of the requirements lock and amend the extra_pip_args array with them.