~ach/hermes unlisted

Tests and fixes for broken tests.
Work on edn mirrors, removing content mirrors and a new builtin.
Make mirror database optional for fetch server.
Use simple edn instead of sqlite for mirror database.
Deprecate content mirror concept.
Don't use empty content mirror as fallback.
Simplify build script
Fail on forwarding errors.
Pluggable sandboxes

Sandboxes are now an external binary that gets run, this can be used to
enable different sandbox engines for package builds. Possible choices
are things like docker, qemu, user mode linux, gvisor and more.

The sandbox binary is specified in the store config, and is either an
absolute path, or a path relative to the hermes-pkgstore binary.
Work on shrinking the package store size.
Index content recursively.
Add a content mirror configuration option.

content_mirror in the hermes config allows
a user to mirror files and content that would
otherwise fail when using an external mirror.
Add notion of build system and other refactoring.
Add placeholder for store path.
Only pass -t to the package build.
Add expect to dev shell.
prepare some debugging flags.
Rework signal handling code.

Reroute signal forwarding via a unix socket,
the goal is to free up stdin to make way for
build debugging.
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