package fetching: catch relative paths that resolve into cache dir

The logic here already caught the case when a dependency path tried to
escape out of the zig cache directory using up directories. However, it
did not catch the case when the relative path tried to reach into a
different path within the zig-cache. For example, if it asked for
"../../../blah" then it would be caught, but if it asked for "../blah"
then it would try to resolve as "zig-cache/p/blah" and probably result
in file-not-found, or perhaps resolve to a different package if someone
inadvertently used a valid package hash instead of "blah".

Now it correctly gives a "dependency path outside project" error,
however, still allows relative paths with up-dirs that were not fetched
via URL.

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