Have Repository::Installation's id change when installing a module

The repository's identity needs to depend on its content so installation of
a module into this repo can automatically invalidate precomp files of repos
nearer the head of the repo chain. Those precomp ids contain the ids of all
repos in the chain.

src/core/CompUnit/Repository/Installation.pm

@@ -2,6 +2,7 @@ class CompUnit::Repository::Installation does CompUnit::Repository::Locally does
     has $!cver = nqp::hllize(nqp::atkey(nqp::gethllsym('perl6', '$COMPILER_CONFIG'), 'version'));
     has %!loaded;
     has $!precomp;
+    has $!id;
 
     submethod BUILD(:$!prefix, :$!lock, :$!WHICH, :$!next-repo) { }
 
@@ -121,7 +122,7 @@ sub MAIN(:$name, :$auth, :$ver, *@, *%) {
         my $has-provides;
         for @files -> $file is copy {
             $file = $is-win ?? ~$file.subst('\\', '/', :g) !! ~$file;
-            my $id = nqp::sha1($file ~ self.id);
+            my $id = nqp::sha1($file ~ self.prefix);
             my $destination = $sources-dir.child($id);
             if [||] @provides>>.ACCEPTS($file) -> $/ {
                 my $name = $/.ast;
@@ -164,7 +165,7 @@ sub MAIN(:$name, :$auth, :$ver, *@, *%) {
         for $d.provides.values.map(*.values[0]<file>) -> $file-id {
             my $source = $sources-dir.child($file-id);
             if $precomp.may-precomp {
-                my $id = nqp::sha1($source ~ self.id);
+                my $id = nqp::sha1($source ~ self.prefix);
                 my $rev-deps-file = ($precomp.store.path($*PERL.compiler.id, $id) ~ '.rev-deps').IO;
                 my @rev-deps = $rev-deps-file.e ?? $rev-deps-file.lines !! ();
 
@@ -175,6 +176,10 @@ sub MAIN(:$name, :$auth, :$ver, *@, *%) {
                 }
             }
         }
+
+        # reset cached id so it's generated again on next access.
+        # identity changes with every installation of a dist.
+        $!id = Any;
         $lock.unlock;
     } ) }
 
@@ -234,7 +239,7 @@ sub MAIN(:$name, :$auth, :$ver, *@, *%) {
                     );
                     my $handle;
                     if $precomp.may-precomp {
-                        my $id = nqp::sha1($loader ~ self.id);
+                        my $id = nqp::sha1($loader ~ self.prefix);
                         if $*W and $*W.is_precompilation_mode {
                             say $id;
                             $handle = $precomp.load($id) or $precomp.precompile($loader, $id);
@@ -260,6 +265,12 @@ sub MAIN(:$name, :$auth, :$ver, *@, *%) {
         nqp::die("Could not find $file in:\n" ~ $*REPO.repo-chain.map(*.Str).join("\n").indent(4));
     }
 
+    method id() {
+        return $!id if $!id;
+        $!id = self.CompUnit::Repository::Locally::id();
+        $!id = nqp::sha1($!id ~ self!dist-dir.dir)
+    }
+
     method short-id() { 'inst' }
 
     method loaded() returns Iterable {