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, 17 insertions, 9 deletions

diff --git a/libbuild2/parser.hxx b/libbuild2/parser.hxx
index 6552114..c55e14f 100644
--- a/libbuild2/parser.hxx
+++ b/libbuild2/parser.hxx
@@ -33,11 +33,17 @@ namespace build2
     void
     parse_buildfile (istream&,
                      const path_name&,
-                     scope& root,
-                     scope& base);
+                     scope* root,
+                     scope& base,
+                     target* = nullptr,
+                     prerequisite* = nullptr);
 
     void
-    parse_buildfile (lexer&, scope& root, scope& base);
+    parse_buildfile (lexer&,
+                     scope* root,
+                     scope& base,
+                     target* = nullptr,
+                     prerequisite* = nullptr);
 
     buildspec
     parse_buildspec (istream&, const path_name&);
@@ -51,7 +57,7 @@ namespace build2
     names
     parse_export_stub (istream& is, const path_name& name, scope& r, scope& b)
     {
-      parse_buildfile (is, name, r, b);
+      parse_buildfile (is, name, &r, b);
       return move (export_value);
     }
 
@@ -99,7 +105,9 @@ namespace build2
     parse_clause (token&, token_type&, bool one = false);
 
     void
-    parse_variable_block (token&, token_type&, const target_type*, string);
+    parse_variable_block (token&, token_type&,
+                          const target_type* = nullptr,
+                          string = string ());
 
     // Ad hoc target names inside < ... >.
     //
@@ -270,14 +278,14 @@ namespace build2
     attributes&
     attributes_top () {return attributes_.back ();}
 
-    // Source a stream optionnaly entering it as a buildfile and performing
-    // the default target processing.
+    // Source a stream optionnaly performing the default target processing.
+    // If the specified path name has a real path, then also enter it as a
+    // buildfile.
     //
     void
     source (istream&,
             const path_name&,
             const location&,
-            bool enter,
             bool default_target);
 
     // The what argument is used in diagnostics (e.g., "expected <what>
@@ -494,7 +502,7 @@ namespace build2
     // Enter buildfile as a target.
     //
     void
-    enter_buildfile (const path_name&);
+    enter_buildfile (const path&);
 
     // Lexer.
     //