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, 21 insertions, 0 deletions

diff --git a/libbuild2/scope.hxx b/libbuild2/scope.hxx
index e1cdf78..e7c2db7 100644
--- a/libbuild2/scope.hxx
+++ b/libbuild2/scope.hxx
@@ -19,6 +19,7 @@
 #include <libbuild2/target-key.hxx>
 #include <libbuild2/target-type.hxx>
 #include <libbuild2/target-state.hxx>
+#include <libbuild2/prerequisite-key.hxx>
 
 #include <libbuild2/export.hxx>
 
@@ -330,6 +331,20 @@ namespace build2
     target_key
     find_target_key (names&, const location&) const;
 
+    // Similar to the find_target_type() but does not complete relative
+    // directories.
+    //
+    pair<const target_type&, optional<string>>
+    find_prerequisite_type (name&, name&, const location&) const;
+
+    // As above, but return a prerequisite key.
+    //
+    prerequisite_key
+    find_prerequisite_key (name&, name&, const location&) const;
+
+    prerequisite_key
+    find_prerequisite_key (names&, const location&) const;
+
     // Dynamically derive a new target type from an existing one. Return the
     // reference to the target type and an indicator of whether it was
     // actually created.
@@ -497,6 +512,12 @@ namespace build2
                               // NULL means no strong amalgamtion.
   };
 
+  inline bool
+  operator== (const scope& x, const scope& y) { return &x == &y; }
+
+  inline bool
+  operator!= (const scope& x, const scope& y) { return !(x == y); }
+
   inline ostream&
   operator<< (ostream& os, const scope& s)
   {