…ion of the file, not the current working directory of Rover (#2119)

As per the title, to support integration with the Language Server we
need to remove a 'quirk' of Rover whereby if you specify a file in the
supergraph.yaml with a relative path, that path is resolved relative to
the location that rover is running in at the time. This doesn't work in
that context so instead we are changing it to resolve relative to the
location of the supergraph.yaml file itself.

I've added a unit test for this and run my own tests and it all seems to
work ok.

In addition I've fixed a test that has been broken for a long time, a
more detailed write up on this test will follow

Successful smoke test run:
https://github.com/apollographql/rover/actions/runs/10795662532/job/29943305762

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---------

Co-authored-by: Aaron <aaronarinder@gmail.com>
Co-authored-by: Dan <danbond@protonmail.com>

Co-authored-by: Brian George <brian@apollographql.com>

Cherry pick changes from
01695b4

Co-authored-by: Dylan Anthony <dylan@apollographql.com>

…#2162)