Replace most uses of Boost.{Filesystem,Process}

With the `ghc::filesystem` dependency from the previous commit. If we
can replace the rest of the Boost.Filesystem dependency then we can get
rid the one nasty runtime dependency we have, and it will make
implementing the chainloading simpler since can reuse more code without
bringing in Boost.

meson.build

@@ -46,8 +46,10 @@ compiler_options = [
   '-fvisibility-inlines-hidden',
   # Disable the use of concepts in Boost.Asio until Boost 1.73 gets released
   # https://github.com/boostorg/asio/issues/312
+  # TODO: Rename after switching to non-Boost ASIO
   '-DBOOST_ASIO_DISABLE_CONCEPTS',
   # Boost.Process's auto detection for vfork() support doesn't seem to work
+  # TODO: Remove after adding our own library
   '-DBOOST_POSIX_HAS_VFORK=1',
   # We use an intrinsic to force flush-to-zero. SSE2 is always enabled in x86_64
   # CPUs, but when we're compiling the 32-bit bitbridge we need to manually add
@@ -242,6 +244,7 @@ else
   bitsery_dep = dependency('bitsery', version : '>=5.2.0')
 endif
 function2_dep = dependency('function2', version : '>=4.0.0')
+ghc_filesystem_dep = dependency('ghc_filesystem', version : '>=1.5.0')
 threads_dep = dependency('threads')
 tomlplusplus_dep = dependency('tomlplusplus', version : '>=2.0.0')
 
@@ -298,6 +301,7 @@ shared_library(
       : boost_filesystem_64bit_dep,
     bitsery_dep,
     dl_dep,
+    ghc_filesystem_dep,
     rt_dep,
     threads_dep,
     tomlplusplus_dep,
@@ -323,6 +327,7 @@ if with_vst3
       bitsery_dep,
       dl_dep,
       function2_dep,
+      ghc_filesystem_dep,
       rt_dep,
       threads_dep,
       tomlplusplus_dep,