pak 0.3.0

• pak functions that used to return tibbles return data frames now.
  While data frames and tibbles are very similar, they are not completely
  compatible. To convert the outputs of pak functions to tibbles call the
  tibble::as_tibble() function on them. If the pillar package is loaded,
  it improves the printing of the returned data frames.

  Relatedly, pak::pak_install_extra() installs pillar now, instead of tibble.

• pak now supports file:// repositories.

• pak now uses HTTP 1.1 to download packages on Linux, in addition to macOS.
  This fixes HTTP issues with some servers (#358).

• New ?ignore-before-r parameter to ignore optional dependencies that
  need a newer R version (r-lib/pkgdepends#243).

• New ?ignore parameter to ignore an optional dependency.

• Allow specifying downstream package parameters with the package=?param
  syntax.

• lockfile_install() now works better for any:: refs, and pak always
  install the version it has planned for.

• System requirement installation is now more robust and works for
  Unix shell expressions (#347).

• CRAN-like resolution is more robust now if a repository is missing
  the usual metadata.

• The lock file is pretty JSON now.

• pak now handles all version requirement types properly:
  '<', '<=', ==, >=, >.

• The dependency solver now uses better heuristics and does not
  (effectively) freeze if multiple repositories have multiple versions of
  the same packages (e.g. RSPM and CRAN)
  (r-lib/pkgdepends#277)