While it may suffice to merge string and non-reserved tags by forming a
union of all tags of dependencies of the same name, this approach fails
to work for the reserved tags. These are now merged such that the most
restrictive tag (meaning sometimes an empty tag) is preserved.

The previous behavior caused essential dependencies to be omitted and
builds to fail in response. E.g., multiple `:fortran` dependencies with
tags `[]`, `[:recommended]`, and `[:optional]` would have been expanded
and merged to `"gcc"` with tags `[:recommended, :optional]`, causing it
to be no longer seen as a required dependency.

Closes Homebrew/homebrew#47040.

Signed-off-by: Martin Afanasjew <martin@afanasjew.de>

The general idea is that merging multiple dependencies of the same name
should produce the strictest tag instead of a (meaningless) union of the
reserved tags. For example, if a dependency is both a `:recommended` and
an `:optional` dependency at different points in the dependency tree,
the resulting merged dependency should be tagged as `:recommended`.
Handle all other reserved tags in the same spirit.

This means that dependencies can be merged but still maintain all
their option names.

Closes Homebrew/homebrew#46916.

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

This test will obviously fail but it does mark that the option names
are not being merged correctly.

Closes Homebrew/homebrew#42354.

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