Implement ad hoc regex pattern rule support

An ad hoc pattern rule consists of a pattern that mimics a dependency
declaration followed by one or more recipes. For example:

exe{~'/(.*)/'}: cxx{~'/\1/'}
{{
  $cxx.path -o $path($>) $path($<[0])
}}

If a pattern matches a dependency declaration of a target, then the recipe is
used to perform the corresponding operation on this target. For example, the
following dependency declaration matches the above pattern which means the
rule's recipe will be used to update this target:

exe{hello}: cxx{hello}

While the following declarations do not match the above pattern:

exe{hello}:   c{hello}  # Type mismatch.
exe{hello}: cxx{howdy}  # Name mismatch.

On the left hand side of `:` in the pattern we can have a single target or an
ad hoc target group. The single target or the first (primary) ad hoc group
member must be a regex pattern (~). The rest of the ad hoc group members can
be patterns or substitutions (^). For example:

<exe{~'/(.*)/'} file{^'/\1.map/'}>: cxx{~'/\1/'}
{{
  $cxx.path -o $path($>[0]) "-Wl,-Map=$path($>[1])" $path($<[0])
}}

On the left hand side of `:` in the pattern we have prerequisites which can
be patterns, substitutions, or non-patterns. For example:

<exe{~'/(.*)/'} file{^'/\1.map/'}>: cxx{~'/\1/'} hxx{^'/\1/'} hxx{common}
{{
  $cxx.path -o $path($>[0]) "-Wl,-Map=$path($>[1])" $path($<[0])
}}

Substitutions on the left hand side of `:` and substitutions and non-patterns
on the right hand side are added to the dependency declaration. For example,
given the above rule and dependency declaration, the effective dependency is
going to be:

<exe{hello} file{hello.map>: cxx{hello} hxx{hello} hxx{common}

1 files changed, 6 insertions, 4 deletions

diff --git a/libbuild2/algorithm.hxx b/libbuild2/algorithm.hxx
index 90159d3..984b786 100644
--- a/libbuild2/algorithm.hxx
+++ b/libbuild2/algorithm.hxx
@@ -111,10 +111,12 @@ namespace build2
 
   // Search for a target identified by the name. The semantics is "as if" we
   // first created a prerequisite based on this name in exactly the same way
-  // as the parser would and then searched based on this prerequisite.
+  // as the parser would and then searched based on this prerequisite. If the
+  // target type is already resolved, then it can be passed as the last
+  // argument.
   //
   LIBBUILD2_SYMEXPORT const target&
-  search (const target&, name, const scope&);
+  search (const target&, name, const scope&, const target_type* = nullptr);
 
   // Return NULL for unknown target types. Note that unlike the above version,
   // these ones can be called during the load and execute phases.
@@ -231,8 +233,8 @@ namespace build2
   LIBBUILD2_SYMEXPORT target&
   add_adhoc_member (target&,
                     const target_type&,
-                    const dir_path& dir,
-                    const dir_path& out,
+                    dir_path dir,
+                    dir_path out,
                     string name);
 
   // If the extension is specified then it is added to the member's target