Fix resolve order of required tools

The order of resolving required tools such is Ninja :

1. Explicit path provided by the user through the QtBuild extension
2. Implicit path resolved by looking up inside Qt installation dir (not the kit but the location where Qt installer installs the tools)
3. Availability of the tool on the PATH environment variable of the host.

Example:

• First: user did not provide the ninja path
• Second: User provided the path Qt installation dir but they did not install Ninja using the Qt online installer
• Third: We check if Ninja is available on PATH, if yes, we use it, else configuration fails with an appropriate message.