Clean up SearchPath

- Better types

- Own header / C++ file pair

- Test factored out methods

- Pass parsed thing around more than strings

Co-authored-by: Robert Hensing <roberth@users.noreply.github.com>

src/libcmd/common-eval-args.cc

@@ -105,7 +105,9 @@ MixEvalArgs::MixEvalArgs()
   )",
         .category = category,
         .labels = {"path"},
-        .handler = {[&](std::string s) { searchPath.push_back(s); }}
+        .handler = {[&](std::string s) {
+            searchPath.elements.emplace_back(SearchPath::Elem::parse(s));
+        }}
     });
 
     addFlag({