* Traverse symlinks when resolving migrations
When enumerating the source directory seeking migration files, `sqlx`
ignores entries that aren't files. This was previously reported as #614
and fixed in #985 but apparently regressed somewhere along the way. This
commit reintroduces the fix from #985 to the current implementation: use
`std::fs::metadata` instead of `std::fs::DirEntry::metadata`. The former
is documented to traverse symlinks; the latter does not.
* add migrations_symlink test
