Pass requirements text directly to parse_requirements

[malikoth]

I have a use case where it may be easier to pass a blob of text representing the contents of a requirements file, rather than a file name (because a file may not actually exist on the FS). Could we refactor this so that you can pass an argument like parse_requirements(contents=my_text_blob) and have it go around the whole process of opening and reading the file?

Alternatively, if it at least supported passing filenames or file-like objects, then I could at least pass in a StringIO object that could be opened.

[di]

This function is designed to replace pip._internal.req.req_file.parse_requirements, which only takes a filename.

When we wrap pip to support this function in pip-api, we need an actual filename to provide to pip on the command line, so pip_api would need to write out these contents to a temporary file in order to make this happen.