package kit: Add API for parsing resolvable expression strings

* Add BPackageInfo::ParseResolvableExpressionString().
* Add BPackageResolvableExpression constructor and SetTo() taking an
  expression string.

headers/os/package/PackageInfo.h

@@ -196,6 +196,10 @@ public:
 									bool revisionIsOptional,
 									BPackageVersion& _version,
 									ParseErrorListener* listener = NULL);
+	static	status_t			ParseResolvableExpressionString(
+									const BString& string,
+									BPackageResolvableExpression& _expression,
+									ParseErrorListener* listener = NULL);
 
 public:
 	static	const char*	const	kElementNames[];

headers/os/package/PackageResolvableExpression.h

@@ -45,12 +45,12 @@ public:
 								BPackageResolvableExpression(
 									const BPackageResolvableExpressionData& data
 									);
+	explicit					BPackageResolvableExpression(
+									const BString& expressionString);
 								BPackageResolvableExpression(
 									const BString& name,
-									BPackageResolvableOperator _op
-										= B_PACKAGE_RESOLVABLE_OP_ENUM_COUNT,
-									const BPackageVersion& version
-										= BPackageVersion());
+									BPackageResolvableOperator _operator,
+									const BPackageVersion& version);
 
 			status_t			InitCheck() const;
 
@@ -60,11 +60,10 @@ public:
 
 			BString				ToString() const;
 
+			status_t			SetTo(const BString& expressionString);
 			void				SetTo(const BString& name,
-									BPackageResolvableOperator _op
-										= B_PACKAGE_RESOLVABLE_OP_ENUM_COUNT,
-									const BPackageVersion& version
-										= BPackageVersion());
+									BPackageResolvableOperator _operator,
+									const BPackageVersion& version);
 			void				Clear();
 
 			bool				Matches(const BPackageVersion& version,