Fixed issue with windows directory separator

[inri13666]

Without this fix, I got on my fresh SF5 project on WIndows following issue

In MappingPass.php line 65:
  Class App\src\Kernel does not exist  

STR:

composer create-project symfony/skeleton
cd skeleton
composer require ongr/elasticsearch-bundle:7.0.*

ENVIRONMENT:
systeminfo | findstr /B /C:"OS Version" /C:"System Type" /C:"System Locale"

OS Version:                10.0.17763 N/A Build 17763
System Type:               x64-based PC
System Locale:             en-us;English (United States)

php -i | findstr /B /C:"PHP Version" /C:"Architecture" /C:"Compiler" /C:"Configure Command" /C:"Zend Extension Build" /C:"Client API library version" /C:"PostgreSQL(libpq)"

PHP Version => 7.4.2
Compiler => Visual C++ 2017
Architecture => x64
Configure Command => cscript /nologo configure.js  "--enable-snapshot-build" "--enable-debug-pack" "--with-pdo-oci=c:\php-snap-build\deps_aux\oracle\x64\instantclient_12_1\sdk,shared" "--with-oci8-12c=c:\php-snap-build\deps_aux\oracle\x64\instantclient_12_1\sdk,shared" "--enable-object-out-dir=../obj/" "--enable-com-dotnet=shared" "--without-analyzer" "--with-pgo"
Zend Extension Build => API320190902,TS,VC15
PHP Version => 7.4.2
Client API library version => mysqlnd 7.4.2
PostgreSQL(libpq) Version => 11.4
PostgreSQL(libpq) Version => 11.4
PostgreSQL(libpq)  => PostgreSQL 11.4, compiled by Visual C++, 64-bit