Before, cache manifest files would have absolute file paths. This is
problematic for two reasons:
* Absolute file paths are not portable. Some operating systems such as
WASI have trouble with them. The files themselves are less portable;
they cannot be migrated from one user's home directory to another's.
And finally they can break due to file paths exceeding maximum path
component size.
* They would prevent some advanced use cases of Zig, where the lib dir
has a different path in a different invocation but is ultimately the
same Zig version and lib directory as before.
This commit adds a new column that specifies the prefix directory for
each file. 0 is an escape hatch and has the previous behavior. The other
two prefixes introduced are zig lib directory, and the cache directory.
This means files in zig-cache manifests can reference files local to
these directories.
In practice, this means it is possible to use a different file path for
the zig lib directory in a subsequent run of zig and have it still take
advantage of the global cache, provided that the files inside remain
unchanged.
closes#13050
21bd13626d