Different types of blockages should be treated differently in MPL

[AcKoucher]

Description

Currently pin access blockages are treated as hard constraints, i.e., area of pin access is inaccessible for std cell clusters, which generates a clear discrepancy between MPL and GPL behavior, as GPL will definitely be able to use an empty area to place std cells.

Suggested Solution

Virtual pin access blockages generated by MPL should work better as soft constraints i.e., clusters of std cells should be allowed to occupy the area of pin access blockage.

Additional Context

See #9214.