Auto merge of #3022 - alexcrichton:add-more-metadata, r=brson

Add a temporary env var to enable hashes in filenames

For rustbuild we need the hashes to exist for all deps, even if they're path
deps, because we care about the actual file names. For example we don't want to
install /usr/lib/libstd.so!

This adds a "secret" environment variable, `__CARGO_DEFAULT_LIB_METADATA` which
re-enables the old behavior of just putting hashes in filenames.

Closes #3005