The two main changes, done together in this commit, are:
* Eliminating most instances of `aux_source_directory()`, replacing
them with explicit file listings; and
* Moving these file listings into respective subdirectories by
representing respective nextpnr components as interface libraries.
In addition, the GUI CMake script tree was simplified since it had
a lot of unused/redundant code.
The `aux_source_directory()` command is not recommended for use by
CMake itself because it misses dependency changes when adding/removing
files, and consequently causes build failures requiring a clean rebuild.
This commit does not touch anything related to architectures/families,
which are very complex and redundant all on their own.
When implementing concepts such as partition pins or deliberately split
nets, there's a need for something that looks like a cell (starts/ends
routing with pins on nets, has timing data) but isn't mapped to a fixed
bel in the architecture, but instead can have pin mappings defined at
runtime.
The PseudoCell allows this by providing an alternate, virtual-function
based API for such cells. When a cell has `pseudo_cell` used, instead of
calling functions such as getBelPinWire, getBelLocation or getCellDelay
in the Arch API; such data is provided by the cell itself, fully
flexible at runtime regardless of arch, via methods on the PseudoCell
implementation.
