FileSystem.pm6

    has %!known; # cache `provides` of META6 when available
    method !matching-file(CompUnit::DependencySpecification $spec) {
        if $spec.from eq 'Perl6' {
            my $name = $spec.short-name;
            return %!loaded{$name} if %!loaded{$name}:exists;

            my $base := $!prefix.child($name.subst(:g, "::", $*SPEC.dir-sep) ~ '.').Str;
            return $base if %seen{$base}:exists;

            my $meta = $!prefix.child('META6.json');
            %!known //= try {
                Rakudo::Internals::JSON.from-json($meta.slurp).hash<provides>;
                CATCH {
                    when JSONException {
                        fail "Invalid JSON found in META6.json";
                    }
                }
            };

            my $found = %!known{$name}:exists && %!known{$name}.IO.f
                ?? %!known{$name}.IO
                !! %extensions<Perl6>.map({ $base ~ $extension }).first(*.IO.e).IO;

            return $base, $found if $found;
        }
        False
    }