Perform post-installation checks if a formula is installed.
Added for brew test-bot to be able to fail on bad Cellar installation.

LanguageModuleDependency.new(:python,...) is deprecated now.
Replace it by
    depends_on :python => ['module' => 'name-on-PyPi']

Note, in the explict form:

    PythonInstalled.new('2.7') => :recommended

the tag :recommended is ignored (not a limitation
of PythonInstalled itself). One solution was to write

    PythonInstalled.new('2.7', [:recommended])

but that is not as beautiful as we like it.

Therefore, now it is possible to:

    depends_on :python => ['2.7', :recommended]

Only the first tag is attempted to be parsed as
a version specifyer "x" or "x.y" or "x.y.z"...

Since e1c62c35465eb6149688c745e4309438c08c410b, we exit early if the
outdated array is empty, so we no longer need to check this before
proceeding.

Only run the `python do ... end` loop once even if
multiple `depends_on :python => 'module-name'` are
present.

As documented in the man page, when given no arguments, `brew search`
will list all formulae. This is different than giving an *empty*
argument.

Closes Homebrew/homebrew#21216.

Fixes Homebrew/homebrew#19475.
Fixes Homebrew/homebrew#21184.

All core formulae have been migrated to .io addresses except
for software maintained by github itself.

Closes Homebrew/homebrew#21163.

Now that this is handled by a Requirement with a default formula, this
code cannot be triggered under normal circumstances.

The advice given has now been moved to the Formula Cookbook on the wiki.

Closes Homebrew/homebrew#21060.

Closes Homebrew/homebrew#21113.

A tapped formula is a ruby file present:

  - in the root of the tap
  - in directory of the tap called Formula
  - in a directory of the tap called HomebrewFormula

And nowhere else. This corrects an overzealous definition of tapped
formula in the updater. (the correct definition has been in Pathname
since e613cbe5783cea2abb8100b56c22126a1ab6b9f2)

Refs Homebrew/homebrew#19743.
Closes Homebrew/homebrew#21087.

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

Refs Homebrew/homebrew#21023.