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, 11 insertions, 2 deletions

diff --git a/libbuild2/module.hxx b/libbuild2/module.hxx
index f5c726e..b2b42e4 100644
--- a/libbuild2/module.hxx
+++ b/libbuild2/module.hxx
@@ -170,9 +170,18 @@ namespace build2
 
   // A wrapper over init_module() to use from other modules that incorporates
   // the <name>.loaded variable check (use init_module() directly to sidestep
-  // this check).
+  // this check). Return a pointer to the pointer to the module instance if it
+  // was both successfully loaded and configured and NULL otherwise (so can be
+  // used as bool).
   //
-  LIBBUILD2_SYMEXPORT bool
+  // Note also that NULL can be returned even of optional is false which
+  // happens if the requested module has already been loaded but failed to
+  // configure. The function could have issued diagnostics but the caller can
+  // normally provide additional information.
+  //
+  // Note: for non-optional modules use the versions below.
+  //
+  LIBBUILD2_SYMEXPORT const shared_ptr<module>*
   load_module (scope& root,
                scope& base,
                const string& name,