lang: Improve cli input handling for files/directories and metadata.yaml

[frebib]

• lang: correctly handle stat(2) errors on directories/files

  Differentiate between ENOENT and other errors returned by stat(2) to
  prevent confusing parse errors when the path provided is a directory
  path. ENOENT is silently ignored as file or directory absence can be
  expected depending on the specified input from the user.

  Handle directory paths without trailing slashes from the user input on
  the command line, correctly disambiguating directories from files with
  the stat(2) result.

  This fixes the rather confusing situation where mgmt would report a
  parse error on the input string of the directory name as if it didn't
  exist, but instead stat(2) just returned another error such as EACCES or
  otherwise. Now correctly report errors that aren't ENOENT.

  Before:

    $ mgmt run lang files
    2020-11-24 20:48:33.350321 I | cli: lang: input from code source: files
    2020-11-24 20:48:33.350389 I | cli: lang: lexing/parsing...
    2020-11-24 20:48:33.350528 I | run: error: cli parse error: could not generate AST: parser: `syntax error: unexpected $end` @1:1
  

  After:

    $ mgmt run lang files
    2020-11-24 20:53:35.500436 I | run: error: cli parse error: could not activate an input parser: stat /home/frebib/mgmt/files/metadata.yaml: permission denied
  

  Signed-off-by: Joe Groocock me@frebib.net

• lang: allow 'mgmt lang run metadata.yaml'

  Handle the case of passing 'metadata.yaml' as an input source to the
  input selector and parse the relative path to the file the same way as
  it would be handled if it were a relative path with a directory
  component, or an absolute path.

  Signed-off-by: Joe Groocock me@frebib.net