Now that this is always called with an explicit mode argument, let's
make sure it stays that way.

Closes Homebrew/homebrew#27642.

Sometimes `Dir[]` is used with single files/directories unnecessarily.

The Homebrew command `switch` will now be completed and installed
fomulas can be completed. The installed versions (the last argument of
`brew switch [formula] [version]`) is not completed at this point.

Closes Homebrew/homebrew#27966.

Signed-off-by: Mike McQuaid <mike@mikemcquaid.com>

Fixes Homebrew/homebrew#27962.

Closes Homebrew/homebrew#27820.

Rationale: our arg refurbishment is normally only turned on when
called via the `make` wrapper, for compatibility reasons. However,
there are numberous places we'd like this to be turned on elsewhere,
like software that builds via `python setup.py` where bad flags from
the system python can be pulled in.

This helper appends 'O' to CCCFG, which enables refurbishment for
all calls of the compiler shims.

Closes Homebrew/homebrew#27949.

Signed-off-by: Jack Nagel <jacknagel@gmail.com>

Closes Homebrew/homebrew#27743.

This method is for internal use only. It is unsuitable for use in
formulae, which should use install_symlink to create relative symlinks.
Thus callers are required to pass a Pathname, not a string, and we can
remove this conditional.

Further, if src is not absolute, then src.relative_path_from(dirname)
will fail. All callers currently pass absolute pathnames. Therefore we
don't need to call expand_path when printing it.

lol inheritance