Add a new "ScanForModuleDependencies" flag.

This controls the Visual Studio "Scan Sources for Module Dependencies" option, which controls how it scans for C++20 modules.  This new flag sets the option in the generated project files.

modules/vstudio/vs2010_vcxproj.lua

@@ -397,6 +397,7 @@
 			m.compileAsWinRT,
 			m.externalWarningLevel,
 			m.externalAngleBrackets,
+			m.scanSourceForModuleDependencies,
 		}
 
 		if cfg.kind == p.STATICLIB then
@@ -2873,6 +2874,15 @@
 	end
 
 
+	function m.scanSourceForModuleDependencies(cfg)
+		if _ACTION >= "vs2019" then
+			if cfg.flags.ScanForModuleDependencies then
+				m.element("ScanSourceForModuleDependencies", nil, "true")
+			end
+		end
+	end
+
+
 	function m.xmlDeclaration()
 		p.xmlUtf8()
 	end

src/_premake_init.lua

@@ -520,6 +520,7 @@
 			"OptimizeSpeed",       -- DEPRECATED
 			"RelativeLinks",
 			"ReleaseRuntime",      -- DEPRECATED
+			"ScanForModuleDependencies",
 			"ShadowedVariables",
 			"StaticRuntime",       -- DEPRECATED
 			"Symbols",             -- DEPRECATED

website/docs/flags.md

@@ -31,6 +31,7 @@ flags { "flag_list" }
 | NoRuntimeChecks       | Disable Visual Studio's [default stack frame and uninitialized variable checks][2] on debug builds. |
 | OmitDefaultLibrary    | Omit the specification of a runtime library in object files.        |
 | RelativeLinks         | Forces the linker to use relative paths to libraries instead of absolute paths. |
+| ScanForModuleDependencies | Enables the Visual Studio `Scan Sources for Module Dependencies` feature for C++20 modules. | Valid for vs2019 and newer. |
 | ShadowedVariables     | Warn when a variable, type declaration, or function is shadowed.    |
 | StaticRuntime         | Perform a static link against the standard runtime libraries.       | Deprecated - use staticruntime "On" instead. |
 | UndefinedIdentifiers | Warn if an undefined identifier is evaluated in an #if directive.   |