[Ninja] Feature of build-tool : Implicit dependency to input '.ninja' file

'Ninja' has implicit dependency on input '.ninja' file.

Let's thing about following example.

< build.ninja >

rule hello

     command = echo ${msg}

build aaaa: hello

      msg = aaaa

build build.ninja: hello

      msg = build.ninja

-----

$ ninja yyyyyyy

[1/1] echo build.ninja

build.ninja

ninja: error: unknown target 'yyyyyyy'

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Ninja try to build input '.ninja' file if rule for the '.ninja' is defined.

And then Ninja try to build given target.