eliminate absolute paths from the build system

[andrewrk]

Absolute paths are generally problematic:

• not portable
• subject to inter-process race conditions
• emplace artificial limitations on lengths of file system paths
• security concerns; incompatible with being explicitly given the available directory handles to work with

For general improvement in robustness as well as progress towards #14286, this issue is to eliminate all absolute paths from the zig build system. Here are some example violations:

https://github.com/ziglang/zig/blob/501a2350ab804f3e5d4253826bcdfb7a3f5d92fb/lib/std/Build/Step/Run.zig#L572-L573

https://github.com/ziglang/zig/blob/501a2350ab804f3e5d4253826bcdfb7a3f5d92fb/lib/std/Build/Step/Run.zig#L642-L656

https://github.com/ziglang/zig/blob/501a2350ab804f3e5d4253826bcdfb7a3f5d92fb/lib/std/Build/Step/WriteFile.zig#L216-L218

There are plenty more.

Instead, this API should be moved from the compiler implementation (Package.Path) to std.Cache.Path:

https://github.com/ziglang/zig/blob/501a2350ab804f3e5d4253826bcdfb7a3f5d92fb/src/Package.zig#L6-L10

Then, Path objects should be passed around rather than strings. This will improve the correctness of a bunch of stuff as well as reduce the amount of unnecessary string manipulation that is happening, and make it easier to implement detection of going outside a directory root.

[ikskuh]

Some general comment from what i've figured: LazyPath is missing a crucial component now that the package manger is there: The path field is relative to some build.zig, but we do not store the relevant builder.

I think we should add some api like fn path(b: *Build, rel: []const u8) LazyPath which constructs a path that is guaranteed to be relative to the given builder.

This way we can safely transport LazyPaths between dependency edges without confusing the build root