Allow requirements_in to be generated - #829

PR Checklist

Please check if your PR fulfills the following requirements:

• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature (please, look at the "Scope of the project" section in the README.md file)
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Documentation content changes
• Other... Please describe:

What is the current behavior?

We generate the requirements_in file from various input files roughly like so:

genrule(
    name = "generate_3.7_x86_requirements",
    srcs = [
        "requirements_base.in.txt",
        "requirements_extra_37.in.txt",
    ],
    outs = ["requirements_3.7_x86.txt"],
    cmd = "cat $(SRCS) > $(OUTS)",
)

compile_pip_requirements(
    name = "compile_requirements_3.7_x86",
    requirements_in = ":requirements_3.7_x86.txt",
    requirements_txt = "requirements_3.7_x86.lock.txt",
)

The current code errors out with a message like this:

Updating common/python/requirements_3.7_x86.lock.txt
Usage: pip_compile.py [OPTIONS] [SRC_FILES]...
Try 'pip_compile.py -h' for help.

Error: Invalid value for '[SRC_FILES]...': Path 'common/python/requirements_3.7_x86.txt' does not exist.

What is the new behavior?

This patch here fixes the issue by resolving the requirements_in path
before the tool cds into the workspace directory.

The downside is that the output of pip still contains the fully resolved
paths on the terminal output. You'll see them in the following example.

$ cd examples/pip_parse
$ bazel run //:requirements.update

The console output will contain the full paths, but the saved
requirements_lock.txt file will not. I could not think of a good way to
also fix up pip's console output.

Does this PR introduce a breaking change?

• Yes
• No

Other information