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, 1 insertions, 25 deletions

diff --git a/libbuild2/prerequisite.hxx b/libbuild2/prerequisite.hxx
index 9448fbc..476ed9d 100644
--- a/libbuild2/prerequisite.hxx
+++ b/libbuild2/prerequisite.hxx
@@ -13,36 +13,12 @@
 #include <libbuild2/variable.hxx>
 #include <libbuild2/target-key.hxx>
 #include <libbuild2/diagnostics.hxx>
+#include <libbuild2/prerequisite-key.hxx>
 
 #include <libbuild2/export.hxx>
 
 namespace build2
 {
-  // Light-weight (by being shallow-pointing) prerequisite key, similar
-  // to (and based on) target key.
-  //
-  // Note that unlike prerequisite, the key is not (necessarily) owned by a
-  // target. So for the key we instead have the base scope of the target that
-  // (would) own it. Note that we assume keys to be ephemeral enough for the
-  // base scope to remain unchanged.
-  //
-  class prerequisite_key
-  {
-  public:
-    using scope_type = build2::scope;
-
-    const optional<project_name>& proj;
-    target_key tk;                // The .dir and .out members can be relative.
-    const scope_type* scope;      // Can be NULL if tk.dir is absolute.
-
-    template <typename T>
-    bool is_a () const {return tk.is_a<T> ();}
-    bool is_a (const target_type& tt) const {return tk.is_a (tt);}
-  };
-
-  LIBBUILD2_SYMEXPORT ostream&
-  operator<< (ostream&, const prerequisite_key&);
-
   // Note that every data member except for the target is immutable (const).
   //
   class LIBBUILD2_SYMEXPORT prerequisite