From type/CompUnit/Repository/Installation

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method install(Distribution $distribution, Bool :$force)

Copies modules into a special location so that they can be loaded afterwards.

:$force will allow installing over an existing distribution that has the same name, auth, api, and ver. Otherwise such a situation will result in Failure.

my $inst-repo = CompUnit::RepositoryRegistry.repository-for-name("site");
my $dist = Distribution::Path.new(...);
$inst-repo.install($dist);