string-util: move version_is_valid() into generic code

While we are at it, replace the sloppy use of filename_is_valid() by the
less sloppy filename_part_is_valid() (as added by the preceeding
commit), since we don#t want to be too restrictive here. (After all,
version strings invalid as standalone filenames might be valid as part
of filenames, and hence we should allow them).

src/basic/string-util.c

@@ -16,6 +16,7 @@
 #include "macro.h"
 #include "memory-util.h"
 #include "memstream-util.h"
+#include "path-util.h"
 #include "string-util.h"
 #include "strv.h"
 #include "terminal-util.h"
@@ -1296,3 +1297,17 @@ char *startswith_strv(const char *string, char **strv) {
 
         return found;
 }
+
+bool version_is_valid(const char *s) {
+        if (isempty(s))
+                return false;
+
+        if (!filename_part_is_valid(s))
+                return false;
+
+        /* This is a superset of the characters used by semver. We additionally allow "," and "_". */
+        if (!in_charset(s, ALPHANUMERICAL ".,_-+"))
+                return false;
+
+        return true;
+}