Fix relative and absolute path handling in lockfiles

[konstin]

Previously, b in the test case would have been incorrectly locked to the path of a. I've moved relative_to into uv-fs since it's now used in two different places.

Previously failing lockfile when a/pyproject.toml and a/b/pyproject.toml exist (not in a workspace) and a was depending on b:

version = 1
requires-python = ">=3.11, <3.13"

[[distribution]]
name = "b"
version = "0.1.0"
source = "directory+/home/konsti/projects/uv/a"
sdist = { path = "/home/konsti/projects/uv/a" }

[[distribution]]
name = "black"
version = "0.1.0"
source = "editable+."
sdist = { path = "." }

[[distribution.dependencies]]
name = "b"
version = "0.1.0"
source = "directory+/home/konsti/projects/uv/a"