controller: Add relative vendor prefix by default

[ermo]

This should allow e.g. /usr/local/bin/boulder to look for its configuration files in /usr/local/bin/../share/

Depends on serpent-os/libmoss#2

[ermo]

Looks like this needs a bit of design work:

• Use the relative tooling binDir of the executed binary for executing sister binaries:
  • onboarding/dev case:
    /usr/local/bin/boulder => binDir = /usr/local/bin/{moss, moss-container}
  • package manager/production case:
    /usr/bin/boulder => binDir = /usr/bin/{moss, moss-container}
• If a config prefix root is specified with -C foo, use configPrefix = foo and assume default vendorPrefix = /usr for config files
• If a config prefix is not specified, use configPrefix = / and set vendorPrefix = binDir/..

NB: In normal use, full paths aren't specified, so the first hit in $PATH will be used. Hence, it will likely be useful if binaries explicitly specify their binDir in a trace statement (to verify assumptions).

Examples:

## pkg manager boulder, automagically deduced relative vendorPrefix
sudo /usr/bin/boulder build stone.yml
# binDir= /usr/bin
# configPrefix = /
# vendorPrefix = /usr/bin/..
# configuration file searchPath = [/usr/bin/../share/boulder, /etc/boulder]

## onboarding boulder, automagically deduced relative vendorPrefix
sudo /usr/local/bin/boulder build stone.yml
# binDir = /usr/local/bin
# configPrefix = /
# vendorPrefix = /usr/local/bin/..
# configuration file searchPath = [/usr/local/bin/../share/boulder, /etc/boulder]

## onboarding boulder, pkg manager configPrefix (bugfix test case)
sudo /usr/local/bin/boulder -C / build stone.yml
# binDir = /usr/local/bin
# configPrefix = /
# vendorPrefix = /usr
# configuration file searchPath = [/usr/share/boulder, /etc/boulder]

## pkg manager boulder, custom configPrefix
sudo /usr/bin/boulder -C /var/lib/machines/sosroot build stone.yml
# binDir = /usr/bin
# configPrefix = /var/lib/machines/sosroot
# vendorPrefix = /var/lib/machines/sosroot/usr
# configuration file searchPath = [/var/lib/machines/sosroot/usr/share/boulder, /var/lib/machines/sosroot/etc/boulder]

@ikeycode: Does ^ sound about right to you?