Name generated BUILD files BUILD.bazel to avoid name conflicts.

[jmillikin-stripe]

This allows new_git_repository to depend on codebases with
pre-existing BUILD files from other tools, or build/ directories
in case-insensitive file systems.

Fixes #3127

[bazel-io]

Can one of the admins verify this patch?

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[katre]

I'll merge this today, thanks for the patch.

[jmillikin-stripe]

(replying to fix the CLA warning)

I signed it!