Modify package info to return remote path

[dapritchard]

Fixes #1057.

As noted in #1057, the main source of the problem is that ess-r-package-info was returning a local path, even when the package was located on a remote system and hence should be addressed using a Tramp filename. I am proposing in this PR that ess-r-package-info should return the remote path and let functions be responsible for converting the remote path into a local path if the local path is what is needed (for example, when providing a path as in input for an R process).

This change does affects a fair amount functions that make use of ess-r-package-info, and some functions had to be modified to continue to work properly after the change. In order to provide some assurance that nothing would get broken by this change, I tried to add some tests for most of the effected functions. In particular, I borrowed some code from the Tramp tests that creates a mock remote file protocol that can be used to test functions with files on remote systems.

The following are some notes that I created for myself to track what functions where effected by the change to ess-r-package-info, and whether anything and/or what needed to be done to incorporate the change.

Functions and variables that are effected by the change to ess-r-package-info

• ess-r-package--info-cache. This results in the variable being changed since
  it is used to cache the result of ess-r-package-info.
• ess-r-package-project. Changes the behavior of the function since it returns
  a list with one of the elements being the remote project path rather than a
  local path. A test was added for this function.
• ess-r-package-name. Doesn’t affect the function since the :name part of
  the plist doesn’t change.
• ess-r-package-source-dirs. The entire reason for changing
  ess-r-package-info is that it fixes a bug in this function. A test was added
  for this function.
• ess-r-package-use-dir. The internals of this function are updated with the
  net result of there being no change to the behavior of the function. A test
  was added for this function.
• ess-r-package-enable-namespaced-evaluation. This shouldn’t change the
  behavior of this function since it just checks for a non-nil value of the
  :root portion of the (ess-r-package-info) plist.
• ess-r-package-eval-linewise. The internals of this function are updated with
  the net result of there being no change to the behavior of the function. A
  test was added for this function.
• ess-r-package-auto-activate. Doesn’t affect the function since it just
  checks for a non-nil value of (ess-r-package-info).

So in summary, ess-r-package--info-cache, ess-r-package-project, and
ess-r-package-source-dirs are effected by the change. Next, each of these
functions or variables are examined for their effect on other functions.

Functions that make use of ess-r-package--info-cache

• ess-r-package-info. The change in ess-r-package--info-cache is required
  for the correct behavior of the function. A test was added for this function.
• ess-project-save-buffers. The change in ess-r-package-info should actually
  have a side benefit of fixing the behavior of this function on remote
  machines. A test was added for this function.
• ess-r-package-re-activate. This function isn’t effected since the function
  simply overwrites the current value of ess-r-package--info-cache.

Functions that make use of ess-r-package-project

• ess-r-package-mode. The function ess-r-package-project is added to the
  hook project-find-functions.
• ess-roxy-enable-in-cpp. Doesn’t affect this function since it just checks for
  a non-nil value returned by ess-r-package-project.
• inferior-ess-r--adjust-startup-directory. This update provides the correct
  behavior since it’s comparing filenames based on the result of
  ess-r-package-project to the value of =default-directory, the latter of
  which has a remote file path when appropriate. A test was added for this
  function.
• ess-r--find-lintr-file. This change should provide the correct behavior
  since it will now look for a .lintr file using a remote path when on a
  remote machine. A test was added for this function.

Functions that make use of ess-r-package-source-dirs

• ess--dbg-find-buffer. This is the function that calls the faulty version of
  ess-r-package-source-dirs, so this should be fixed now.