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:

zig/lib/std/Build/Step/Run.zig

Lines 572 to 573 in 501a235

	const output_path = try b.cache_root.join(arena, &output_components);
	placeholder.output.generated_file.path = output_path;

zig/lib/std/Build/Step/Run.zig

Lines 642 to 656 in 501a235

	placeholder.output.generated_file.path = try cache_root.join(arena, &.{
	"o", digest, placeholder.output.basename,
	});
	}
	if (captured_stdout) |output| {
	output.generated_file.path = try cache_root.join(arena, &.{
	"o", digest, output.basename,
	});
	}
	if (captured_stderr) |output| {
	output.generated_file.path = try cache_root.join(arena, &.{
	"o", digest, output.basename,
	});

zig/lib/std/Build/Step/WriteFile.zig

Lines 216 to 218 in 501a235

	file.generated_file.path = try b.cache_root.join(b.allocator, &.{
	"o", &digest, file.sub_path,
	});

There are plenty more.

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

zig/src/Package.zig

Lines 6 to 10 in 501a235

	pub const Path = struct {
	root_dir: Cache.Directory,
	/// The path, relative to the root dir, that this `Path` represents.
	/// Empty string means the root_dir is the path.
	sub_path: []const u8 = "",

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