CMake: rationalize and refactor build system.

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.

json/CMakeLists.txt

@@ -0,0 +1,8 @@
+add_library(nextpnr_json INTERFACE)
+
+target_include_directories(nextpnr_json INTERFACE .)
+
+target_sources(nextpnr_json PUBLIC
+    jsonwrite.cc
+    jsonwrite.h
+)