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".
