If passed, it will attempt to relative-ize the resulting URL (if possible).

Add helper: fragment_string

Tests:

- abs_ireq_preserves_source_ireqs
- format_requirement_impossible_relative_path_becomes_absolute (Windows-only)

Also, ensure copy_install_requirement preserves our custom attributes.

Specifically ones which start like file:///C:/

Ensure source ireqs' comes_from attr is credited in annotations

This brings with it more involved string manipulation

If passed, from_dir will be used instead of CWD to interpret relative paths.

Special care is taken to guarantee _was_relative in the yielded ireqs,
if originally relative.

When calling format_requirement,
_was_relative's presence now determines what's passed as from_dir:
either CWD or None (None leads to an absolute path output)

For ths to work we also re-assign the _was_relative attr
to ireqs generated from candidates in the backtracking resolver.

With test, and needed changes:

- Use this flag to decide what to pass as from_dir to parse_requirements
- Add init param to OutputWriter: write_relative_to_output
- Add corresponding from_dir logic and use to OutputWriter._format_requirement
- Add test_write_relative_to_output which runs with the new flag both on and off

This ensures absolute intact paths for git+file: reqs

With test_sync_relative_path

If the reqs in the output were written relative to that output file
(pip-compile --write-relative-to-output), then it's wise for pip-sync
to interpret reqs it finds in there from the location of the output file.

This is now possible with pip-sync --read-relative-to-input.

To be clear: pip-sync's "input" is pip-compile's "output."

This fixes at least two problems, for which tests are added.

And add tests via parameterization

Fixes jazzband#1107

e.g. ../../rawr#egg=rawr-cli

This fixes false-positive _was_relative assignments
for some direct references like:

ptrender[yaml] @ file:///home/andy/Code/ptrender

An ireq's extras may be any Collection,
not necessarily a set,
so we can't rely on set operators working implicity.

As we always want to set that True

Based on findings at jazzband#1329 (comment)