fn disabling_file(vroot string) string
disabling_file returns the path to a file, which if present, will disable the automatic recompilation
that V attempts, when it detects that itself is newer than a tool .v source file.
That file is intended to be made by
touch cmd/tools/.disable_autorecompilation by package managers, so that tools like
v up and
v self will not work anymore, instead they will direct users to install
V from source.
fn must_be_enabled(vroot string, error_message string)
must_be_enabled is intended to be used by tools like
v self and
v up, to abort them
early, when they detect that the V installation is part of a distro package, that has disabled autorecompilation.