build/LazyPath: Add dirname

[abhinav]

Adds a variant to the LazyPath union representing a parent directory
of a generated path.

const LazyPath = union(enum) {
    generated_dirname: struct {
        generated: *const GeneratedFile,
        up: usize,
    },
    // ...
}

These can be constructed with the new method:

pub fn dirname(self: LazyPath) LazyPath

For the cases where the LazyPath is already known
(.path, .cwd_relative, and dependency)
this is evaluated right away.
For dirnames of generated files and their dirnames,
this is evaluated at getPath time.

dirname calls can be chained, but for safety,
they are not allowed to escape outside a root
defined for each case:

• path: This is relative to the build root,
  so dirname can't escape outside the build root.
• generated: Can't escape the zig-cache.
• cwd_relative: This can be a relative or absolute path.
  If relative, can't escape the current directory,
  and if absolute, can't go beyond root (/).
• dependency: Can't escape the dependency's root directory.

Testing:
I've included a standalone case for many of the happy cases.
I couldn't find an easy way to test the negatives, though,
because tests cannot yet expect panics.

Resolves #17411

[abhinav]

Ah, I assumed "zig-cache" as the basename in that test.

https://github.com/ziglang/zig/actions/runs/7324898729/job/19999209921?pr=18371#step:3:124

[abhinav]

Build was green but there was a small conflict following #18160 which had to be resolved.

[ikskuh]

Definitly a better solution than #16803. Thanks for taking care of that topic!

[andrewrk]

Thanks, nice work.

In the future I want to improve the implementation by moving away from absolute file paths. I took the time to write up that issue here: #18450

However, in the meantime, this is an improvement, and that issue can be a follow-up change. Thanks to your new tests, it will be easy to make sure that the feature continues to work at that time.