don't rename *.gyp files

[mikemorris]

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Fixes https://github.com/npm/npm/issues/10243, refs https://github.com/npm/read-package-json/pull/52

Currently, *.gyp files are all renamed to binding.gyp if a package.json file exists in the same directory (potentially clobbering each other if multiple *.gyp files exist). This can be problematic for packages which use a standalone gyp build system outside of just node-gyp. This patch removes the current behavior and adds a test case to ensure that main.gyp will not be renamed.

SEMVER MAJOR

/cc @othiym23