build/LazyPath: Add dirname (#18371)

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

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

These can be constructed with the new method:

```zig
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.

0 files changed, 0 insertions, 0 deletions