targets.jam (and project.jam, property-set.jam) review

* fixing some comments
* fixed regex and condition in resolve-reference
* removed common-properties2 rule
* fixed regex in basic-target.compute-usage-requirements
* fixed xusage-requirements in basic-target.compute-usage-requirements now is a local variable instead of a class attribute
* removed all checking for '@error', not need anymore since 0419f26 (2003-09-26), this is also done in build/project.jam and build/property-set.jam
* emit error using errors module in typed-target.construct
* removed unused imports
* moved .debug-building from root module to targets
* fixed checking for generate-really return value
* in main-target.generate; since [ property-set.empty ] always evaluate to true it is better to avoid returning it instead of writing if $(r) = [ property-set.empty ]

core_parallel_actions.py review

* fixed core_parallel_actions.py test logic
* reduced concurrency on core_parallel_actions.py

Author: paolopas

src/build/ac.jam

@@ -141,6 +141,7 @@ class ac-library : basic-target
     import configure ;
     import config-cache ;
     import os ;
+    import project ;
 
     rule __init__ ( name : project : requirements * : include-path ? : library-path ? : library-name ? )
     {

src/build/project.jam

@@ -968,18 +968,9 @@ class project-attributes
         }
         else if $(attribute) = "requirements"
         {
-            local result = [ property-set.refine-from-user-input
+            self.requirements = [ property-set.refine-from-user-input
                 $(self.requirements) : $(specification)
                 : $(self.project-module) : $(self.location) ] ;
-
-            if $(result[1]) = "@error"
-            {
-                import errors : error : errors.error ;
-                errors.error Requirements for project at '$(self.location)'
-                    conflict with parent's. : "Explanation:" $(result[2-]) ;
-            }
-
-            self.requirements = $(result) ;
         }
         else if $(attribute) = "usage-requirements"
         {

src/build/property-set.jam

@@ -183,15 +183,8 @@ class property-set
     {
         if ! $(self.refined.$(ps))
         {
-            local r = [ property.refine $(self.raw) : [ $(ps).raw ] ] ;
-            if $(r[1]) != "@error"
-            {
-                self.refined.$(ps) = [ property-set.create $(r) ] ;
-            }
-            else
-            {
-                self.refined.$(ps) = $(r) ;
-            }
+            self.refined.$(ps) = [ property-set.create
+                [ property.refine $(self.raw) : [ $(ps).raw ] ] ] ;
         }
         return $(self.refined.$(ps)) ;
     }