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)"

4 files changed, 10 insertions, 22 deletions

diff --git a/libbuild2/cc/common.cxx b/libbuild2/cc/common.cxx
index 99de66e..cf6d546 100644
--- a/libbuild2/cc/common.cxx
+++ b/libbuild2/cc/common.cxx
@@ -465,20 +465,8 @@ namespace build2
       {
         // This is import.
         //
-        name n (cn);
-        auto rp (s.find_target_type (n, location ())); // Note: changes name.
-        const target_type* tt (rp.first);
-        optional<string>& ext (rp.second);
-
-        if (tt == nullptr)
-          fail << "unknown target type '" << n.type << "' in library " << n;
-
-        // @@ OUT: for now we assume out is undetermined, just like in
-        // search (name, scope).
-        //
-        dir_path out;
-
-        prerequisite_key pk {n.proj, {tt, &n.dir, &out, &n.value, ext}, &s};
+        name n (cn), o; // Note: find_prerequisite_key() changes name.
+        prerequisite_key pk (s.find_prerequisite_key (n, o, location ()));
         xt = search_library_existing (a, sysd, usrd, pk);
 
         if (xt == nullptr)
diff --git a/libbuild2/cc/compile-rule.cxx b/libbuild2/cc/compile-rule.cxx
index a8916cf..8b082cc 100644
--- a/libbuild2/cc/compile-rule.cxx
+++ b/libbuild2/cc/compile-rule.cxx
@@ -696,8 +696,8 @@ namespace build2
           continue;
 
         // A dependency on a library is there so that we can get its
-        // *.export.poptions, modules, etc. This is the library
-        // meta-information protocol. See also append_lib_options().
+        // *.export.poptions, modules, etc. This is the library metadata
+        // protocol. See also append_lib_options().
         //
         if (pi == include_type::normal &&
             (p.is_a<libx> () ||
@@ -4871,7 +4871,7 @@ namespace build2
       //
       // For (direct) library prerequisites, check their prerequisite bmi{}s
       // (which should be searched and matched with module names discovered;
-      // see the library meta-information protocol for details).
+      // see the library metadata protocol for details).
       //
       // For our own bmi{} prerequisites, checking if each (better) matches
       // any of the imports.
diff --git a/libbuild2/cc/init.cxx b/libbuild2/cc/init.cxx
index 8d66376..2a0dbd2 100644
--- a/libbuild2/cc/init.cxx
+++ b/libbuild2/cc/init.cxx
@@ -399,7 +399,7 @@ namespace build2
 
       // We only support root loading (which means there can only be one).
       //
-      if (&rs != &bs)
+      if (rs != bs)
         fail (loc) << m << " module must be loaded in project root";
 
       // We want to order the loading to match what user specified on the
diff --git a/libbuild2/cc/utility.hxx b/libbuild2/cc/utility.hxx
index fa9f165..017765b 100644
--- a/libbuild2/cc/utility.hxx
+++ b/libbuild2/cc/utility.hxx
@@ -41,10 +41,10 @@ namespace build2
     //
     // The reason we pass scope and not the target is because this function is
     // called not only for exe/lib but also for obj as part of the library
-    // meta-information protocol implementation. Normally the bin.*.lib values
-    // will be project-wide. With this scheme they can be customized on the
-    // per-directory basis but not per-target which means all exe/lib in the
-    // same directory have to have the same link order.
+    // metadata protocol implementation. Normally the bin.*.lib values will be
+    // project-wide. With this scheme they can be customized on the per-
+    // directory basis but not per-target which means all exe/lib in the same
+    // directory have to have the same link order.
     //
     lorder
     link_order (const scope& base, otype);