Build.Cache.Path: fix `resolvePosix` empty `sub_path`

This function is sometimes used to assume a canonical representation of
a path. However, when the `Path` referred to `root_dir` itself, this
function previously resolved `sub_path` to ".", which is incorrect; per
doc comments, it should set `sub_path` to "".

This fix ultimately didn't solve what I was trying to solve, though I'm
still PRing it, because it's still *correct*. The background to this
commit is quite interesting and worth briefly discussing.

I originally worked on this to try and fix a bug in the build system,
where if the root package (i.e. the one you `zig build`) depends on
package X which itself depends back on the root package (through a
`.path` dependency), invalid dependency modules are generated. I hit
this case working on ziglang/translate-c, which wants to depend on
"examples" (similar to the Zig compiler's "standalone" test cases) which
themselves depend back on the translate-c package. However, after this
patch just turned that error into another, I realised that this case
simply cannot work, because `std.Build` needs to eagerly execute build
scripts at `dependency` calls to learn which artifacts, modules, etc,
exist.

...at least, that's how the build system is currently designed. One can
imagine a world where `dependency` sort of "queues" the call, `artifact`
and `module` etc just pretend that the thing exists, and all configure
functions are called non-recursively by the runner. The downside is that
it becomes impossible to query state set by a dependency's configure
script. For instance, if a dependency exposes an artifact, it would
become impossible to get that artifact's resolved target in the
configure phase. However, as well as allowing recursive package imports
(which are certainly kinda nifty), it would also make lazy dependencies
far more useful! Right now, lazy dependencies only really work if you
use options (`std.Build.option`) to block their usage, since any call to
`lazyDependency` causes the dependency to be fetched. However, if we
made this change, lazy dependencies could be made far more versatile by
only fetching them *if the final step plan requires them*. I'm not 100%
sure if this is a good idea or not, but I might open an issue for it
soon.

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