1. Running `brew linkage some_package` does not set the cache.
2. Running `brew linkage --cached some_package` when `DatabaseCache.empty?` returns `true` should build the cache.
3. Running `brew linkage --cached some_package` when `DatabaseCache.empty?` returns `false` should use the cache.

Lazily load db of type `DBM` instance variable for `DatabaseCache` so the corresponding database file isn't created in the `.use` block for a `DatabaseCache`.

Made `DatabaseCache.new` private, and changes instances in code that call it to use `DatabaseCache.use` instead.

Use cache if `HOMEBREW_LINKAGE_CACHE` exists in `formula_cellar_checks` and `formula`. Make functions private in `LinkageChecker`.

Ensure that a non-zero exit code is set both for missing random dylibs
and random missing dependencies.

Additionally, while we are here, drastically trim down the public
interface for this class to the bare minimum and allow getting the
output from `display_test_output` as a variable.

Fixes issue mentioned by @ilovezfs in:
https://github.com/Homebrew/brew/pull/3940#issuecomment-383794520

No reason to have it in macOS class and keeps consistent output with
`brew info` on Linux.

The `cask` attribute doesn't make as much sense on Linux but can be
ignored there. The advantage of this change is that (like #4086) it
allows figuring out the relevant cask for a formulae requirement on a
Linux machine.

Fix various circular requirements, method redefinitions, etc.

A bunch of these were needed before superenv, environment filtering or
on now long-unsupported versions of macOS.

Changed default behavior of `brew linkage` command to build cache instead of using cached output. Cached output may be printed with `--cached` flag.

This reverts commit fa5245f7.

We don't currently have a LinkageChecker on Linux, so can't do this.

Added flag to `flush_cache` in `LinkageChecker`. Format ruby hash as JSON string before storing in `dbm`.

Removed redundant documentation, use database_cache as a block, and use symbolic keys over string keys in function calls.

Defaulting zip_dep_if_needed(tags) and bzip2_dep_if_needed(tags)
methods to those on Linux and overriding them on macOS.

On some systems identified as Linux, zip and bzip2 might
not be available. Therefore, on such platforms we add them
unconditionally as dependencies when required. On Mac, these
dependencies are always satisfied.

Remove more Requirement logic to enable future removal of default
formula logic. Also, output deprecations, convert symbol requirement
usage to deps and simplify the compatibility code for the direct
Requirement usage.

These rely on `default_formula` to be useful and are better following
the `*_dep_if_needed` pattern.

Move ld64_dependency.rb to os/mac/.

These are ones that were either already deprecated due to audit rules
or are just a simple `which` with a `default_formula` so should just
be a dependency.

Fix the warning: already initialized constant X11Requirement