Rework tool importation along with cli module

Specifically, now config.<tool> (like config.cli) is handled by the import
machinery (it is like a shorter alias for config.import.<tool>.<tool>.exe
that we already had). And the cli module now uses that instead of custom
logic.

This also adds support for uniform tool metadata extraction that is handled by
the import machinery. As a result, a tool that follows the "build2 way" can be
imported with metadata by the buildfile and/or corresponding module without
any tool-specific code or brittleness associated with parsing --version or
similar outputs. See the cli tool/module for details.

Finally, two new flavors of the import directive are now supported: import!
triggers immediate importation skipping any rule-specific logic while import?
is optional import (analogous to using?). Note that optional import is always
immediate. There is also the import-specific metadata attribute which can be
specified for these two import flavors in order to trigger metadata
importation. For example:

import? [metadata] cli = cli%exe{cli}

if ($cli != [null])
  info "cli version $($cli:cli.version)"

1 files changed, 43 insertions, 9 deletions

diff --git a/libbuild2/scope.cxx b/libbuild2/scope.cxx
index 842b839..92fc12c 100644
--- a/libbuild2/scope.cxx
+++ b/libbuild2/scope.cxx
@@ -738,7 +738,7 @@ namespace build2
     auto r (find_target_type (n, loc));
 
     if (r.first == nullptr)
-      fail (loc) << "unknown target type " << n.type;
+      fail (loc) << "unknown target type " << n.type << " in " << n;
 
     bool src (n.pair); // If out-qualified, then it is from src.
     if (src)
@@ -784,20 +784,54 @@ namespace build2
       if (n == (ns[0].pair ? 2 : 1))
       {
         name dummy;
-        target_key r (find_target_key (ns[0], n == 1 ? dummy : ns[1], loc));
-
-        return target_key {
-          r.type,
-          r.dir,
-          n == 1 ? &empty_dir_path : r.out,
-          r.name,
-          move (r.ext)};
+        return find_target_key (ns[0], n == 1 ? dummy : ns[1], loc);
       }
     }
 
     fail (loc) << "invalid target name: " << ns << endf;
   }
 
+  pair<const target_type&, optional<string>> scope::
+  find_prerequisite_type (name& n, name& o, const location& loc) const
+  {
+    auto r (find_target_type (n, loc));
+
+    if (r.first == nullptr)
+      fail (loc) << "unknown target type " << n.type << " in " << n;
+
+    if (n.pair) // If out-qualified, then it is from src.
+    {
+      assert (n.pair == '@');
+
+      if (!o.directory ())
+        fail (loc) << "expected directory after '@'";
+    }
+
+    if (!n.dir.empty ())
+      n.dir.normalize (false, true); // Current dir collapses to an empty one.
+
+    if (!o.dir.empty ())
+      o.dir.normalize (false, true); // Ditto.
+
+    return pair<const target_type&, optional<string>> (
+      *r.first, move (r.second));
+  }
+
+  prerequisite_key scope::
+  find_prerequisite_key (names& ns, const location& loc) const
+  {
+    if (size_t n = ns.size ())
+    {
+      if (n == (ns[0].pair ? 2 : 1))
+      {
+        name dummy;
+        return find_prerequisite_key (ns[0], n == 1 ? dummy : ns[1], loc);
+      }
+    }
+
+    fail (loc) << "invalid prerequisite name: " << ns << endf;
+  }
+
   static target*
   derived_tt_factory (context& c,
                       const target_type& t, dir_path d, dir_path o, string n)