Do not use a mixture of plugins{} and buildscript{}

Summary:
In our build logic we're mixing `plugins{}` and `buildscript{}`
which have unpredictable side-effect on the build classpath.

I'm moving over everything to use `plugins{}`. This is possible now
that we don't use build from source for New Architecture anymore.

Changelog:
[Internal] [Changed] - Do not use a mixture of plugins{} and buildscript{}

allow-large-files

Reviewed By: cipolleschi

Differential Revision: D43186768

fbshipit-source-id: dcd115bd9d7aadf5cb837b3a28598e274a092873