Implement `uv run --directory` (#5566)
## Summary
uv run --directory <path> means that one doesn't have to change to a
project's directory to run programs from it. It makes it possible to use
projects as if they are tool installations.
To support this, first the code reading .python-version was updated so
that
it can read such markers outside the current directory. Note the minor
change this causes (if I'm right), described in the commit.
## Test Plan
One test has been added.
## --directory
Not sure what the name of the argument should be, but it's following uv
sync's directory for now.
Other alternatives could be "--project". Uv run and uv tool run should
probably find common agreement on this (relevant for project-locked
tools).
I've implemented this same change in Rye, some time ago, and then we
went
with --pyproject `<`path to pyproject.toml file`>`. I think using
pyproject.toml file path and not directory was probably a mistake, an
overgeneralization one doesn't need.
