I think this will work on linux/mac, not windows obviously. But the essence is that running `bazel run -c opt //installers/local:install` will create:
Symlink: /usr/bin/carbon-explorer -> /usr/lib/carbon/carbon
BusyBox-style binary: /usr/lib/carbon/carbon
File: /usr/lib/carbon/data/prelude.carbon
And then just running `carbon-explorer foo.carbon` with no flags (in particular, not --prelude) will work.
Since this removes the need for prelude_file in most cases, I'm removing the relative path logic added in #1179 -- it would otherwise conflict with what I'm doing here. I *think* overall this will result in something even simpler and more reliable for compiler explorer to use, and hopefully with enough flexibility that it's easy for us to change how it's implemented later without breaking much.
