As part of this:
* add std.process.cleanExit. closes#6395
- use it in several places
* adjust the alignment of text in `zig build --help` menu
* Cache: support the concept of "unhit" so that we properly keep track
of the cache when we find out using the secondary hash that the cache
"hit" was actually a miss. Use this to fix false negatives of caching
of stage1 build artifacts.
* fix not deleting the symlink hash for stage1 build artifacts causing
false positives.
* implement support for Package arguments in stage1 build artifacts
* update and add missing usage text
* add --override-lib-dir and --enable-cache CLI options
- `--enable-cache` takes the place of `--cache on`
* CLI supports -femit-bin=foo combined with --enable-cache to do an
"update file" operation. --enable-cache without that argument
will build the output into a cache directory and then print the path
to stdout (matching master branch behavior).
* errors surfacing from main() now print "error: Foo" instead of
"error: error.Foo".
This change was mostly made with `zig fmt` and this also modified some whitespace. Note that in some files, `zig fmt` produced incorrect code, so the change was made manually.
Thanks to the Windows Process Environment Block, it is possible to
obtain handles to the standard input, output, and error streams without
possibility of failure.
