Build libweaver from supplied sources rather than downloading it.

libweaver is not a real external package: it is five source files lifted out of
isledecomp/SIEdit at a pinned commit, which 3rdparty/CMakeLists.txt compiles
into a static library itself. Upstream only does that when DOWNLOAD_DEPENDENCIES
is on, and otherwise expects find_package(libweaver) to succeed, which nothing
provides.

The download cannot simply be redirected: FetchContent_Populate() called with
the arguments inline, rather than with a name declared earlier, is a direct
populate that ignores both FETCHCONTENT_SOURCE_DIR_<name> and
FETCHCONTENT_FULLY_DISCONNECTED, so it fetches over the network at configure
time regardless. Drop it and take the source directory as a substituted path
instead.

--- a/3rdparty/CMakeLists.txt
+++ b/3rdparty/CMakeLists.txt
@@ -57,13 +57,11 @@
 )
 target_include_directories(libsmacker PUBLIC ${libsmacker_SOURCE_DIR})
 
-if(DOWNLOAD_DEPENDENCIES)
-    include(FetchContent)
-    FetchContent_Populate(
-        libweaver
-        URL https://github.com/isledecomp/SIEdit/archive/abb6d47139d7a03c5c1bad9b89e2267849e27904.tar.gz
-        URL_MD5 d786156fd9a2c5c755e27fd5c5a9ee28
-    )
+# libweaver is not a real external package: it is five sources lifted out of
+# isledecomp/SIEdit, which this file compiles into a static library itself.
+# The sources are supplied out of band, so no download is needed.
+set(libweaver_SOURCE_DIR "@LIBWEAVER_SOURCE_DIR@")
+if(TRUE)
     add_library(libweaver STATIC
         ${libweaver_SOURCE_DIR}/lib/core.cpp
         ${libweaver_SOURCE_DIR}/lib/file.cpp
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -303,7 +303,7 @@
     target_link_libraries(Isle::iniparser INTERFACE iniparser-static)
   endif()
 
-  find_package(libweaver REQUIRED)
+  # libweaver is built from vendored sources in 3rdparty/ instead.
 endif()
 
 set(CMAKE_EXPORT_COMPILE_COMMANDS ON)