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, 3 insertions, 3 deletions

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.