⏪ add MQT::CorePython target back (#541)

## Description

As the integration of the mqt-core Python package into the top-level
tools turned out to be a much lengthier process than imagined, this PR
adds back the `MQT::CorePython` target that was previously used to read
in Qiskit objects.
This should allow development in the top-level libraries to continue
without waiting for the remaining issues with the Python package being
resolved.

## Checklist:

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- [x] The pull request only contains commits that are related to it.
- [x] I have added appropriate tests and documentation.
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- [x] The pull request introduces no new warnings and follows the
project's style guidelines.

---------

Signed-off-by: burgholzer <burgholzer@me.com>

include/mqt-core/python/qiskit/QuantumCircuit.hpp

@@ -0,0 +1,49 @@
+#pragma once
+
+#include "pybind11/pybind11.h"
+#include "pybind11/pytypes.h"
+
+#include <regex>
+#include <type_traits>
+#include <variant>
+
+namespace py = pybind11;
+
+#include "QuantumComputation.hpp"
+
+namespace qc::qiskit {
+using namespace pybind11::literals;
+
+class QuantumCircuit {
+public:
+  static void import(QuantumComputation& qc, const py::object& circ);
+
+protected:
+  static void emplaceOperation(QuantumComputation& qc,
+                               const py::object& instruction,
+                               const py::list& qargs, const py::list& cargs,
+                               const py::list& params, const py::dict& qubitMap,
+                               const py::dict& clbitMap);
+
+  static SymbolOrNumber parseSymbolicExpr(const py::object& pyExpr);
+
+  static SymbolOrNumber parseParam(const py::object& param);
+
+  static void addOperation(QuantumComputation& qc, OpType type,
+                           const py::list& qargs, const py::list& params,
+                           const py::dict& qubitMap);
+
+  static void addTwoTargetOperation(QuantumComputation& qc, OpType type,
+                                    const py::list& qargs,
+                                    const py::list& params,
+                                    const py::dict& qubitMap);
+
+  static void importDefinition(QuantumComputation& qc, const py::object& circ,
+                               const py::list& qargs, const py::list& cargs,
+                               const py::dict& qubitMap,
+                               const py::dict& clbitMap);
+
+  static void importInitialLayout(QuantumComputation& qc,
+                                  const py::object& circ);
+};
+} // namespace qc::qiskit

src/CMakeLists.txt

@@ -144,6 +144,29 @@ add_subdirectory(zx)
 # add ECC library
 add_subdirectory(ecc)
 
+# ** Note ** The following target will soon be removed from the project. All top-level projects
+# should switch to using the mqt-core Python package.
+if(BINDINGS AND NOT TARGET mqt-core-python)
+  # add Python interface library
+  add_library(
+    ${MQT_CORE_TARGET_NAME}-python ${MQT_CORE_INCLUDE_BUILD_DIR}/python/qiskit/QuantumCircuit.hpp
+                                   python/qiskit/QuantumCircuit.cpp)
+
+  # link with main project library and pybind11 libraries
+  target_link_libraries(${MQT_CORE_TARGET_NAME}-python PUBLIC MQT::Core pybind11::pybind11)
+  target_link_libraries(${MQT_CORE_TARGET_NAME}-python PRIVATE MQT::ProjectOptions
+                                                               MQT::ProjectWarnings)
+
+  # add MQT alias
+  add_library(MQT::CorePython ALIAS ${MQT_CORE_TARGET_NAME}-python)
+  set_target_properties(
+    ${MQT_CORE_TARGET_NAME}-python
+    PROPERTIES VERSION ${PROJECT_VERSION}
+               SOVERSION ${PROJECT_VERSION_MAJOR}.${PROJECT_VERSION_MINOR}
+               EXPORT_NAME CorePython)
+  list(APPEND MQT_CORE_TARGETS ${MQT_CORE_TARGET_NAME}-python)
+endif()
+
 if(BUILD_MQT_CORE_BINDINGS)
   add_subdirectory(python)
 endif()