Support constraints on interior signals in nextpnr

[whitequark]

This is technically a bug in all cases, but it is not actually visible in most circumstances, as the signals that appear in constraint files are always ports, ports are assigned names first, and port names are never duplicates when returned by the platform layer. However, it would break most attempts to add interior frequency constraints, like for PLL output.

A complication is that it is currently not possible to reliably add constraints on interior signals in nextpnr (and even ports became reliable only after #71).

[whitequark]

Since 8c30147, constraints on interior signals are supported if the toolchain supports them.

[whitequark]

This should be fixed in YosysHQ/nextpnr#329, but needs some adjustments to vendor code.

[whitequark]

cc @dlharmon