meson: add src/ folder

This creates a conventional structure of src/ and test/ which helps to
separate top-level files such as meson.build, OWNERS, ...
from implementation files.

Git history of individual files is still accessible via e.g.
```
git log --follow -- src/thresholds.hpp
```

Tested: code compiles.

Change-Id: Ifff8b3e70437bc6a25cd6f65afd07d8a563d1a8c
Signed-off-by: Alexander Hansen <alexander.hansen@9elements.com>
diff --git a/meson.build b/meson.build
index fc8ca68..b2d4227 100644
--- a/meson.build
+++ b/meson.build
@@ -19,25 +19,7 @@
     exprtk = declare_dependency(include_directories: 'subprojects/exprtk')
 endif
 
-executable(
-    'virtual-sensor',
-    [
-        'calculate.cpp',
-        'dbusSensor.cpp',
-        'dbusUtils.cpp',
-        'main.cpp',
-        'virtualSensor.cpp',
-    ],
-    dependencies: [
-        dependency('nlohmann_json', include_type: 'system'),
-        dependency('phosphor-dbus-interfaces'),
-        dependency('phosphor-logging'),
-        dependency('sdbusplus'),
-        exprtk,
-    ],
-    install: true,
-    install_dir: get_option('libexecdir') / meson.project_name(),
-)
+subdir('src')
 
 packagedir = join_paths(
     get_option('prefix'),
@@ -58,6 +40,8 @@
     install_dir: systemd.get_variable('systemd_system_unit_dir'),
 )
 
+src_inc = include_directories('src')
+
 build_tests = get_option('tests')
 if build_tests.allowed()
     subdir('test')