MPL: Reintroduce ability to handle FIXED pins

[AcKoucher]

Description

With the new IO Clusters, MPL doesn't have the ability to handle pre-placed pins as it relies uniquely on the IOs constraints in ORFS. However, other types of flow don't benefit from this approach.

Suggested Solution

Bundle pre-placed pins into clusters of bundled pins as the original implementation used to.

Additional Context

No response

[AcKoucher]

@gadfort FYI