`Bundler::LockfileParser#initialize` will parse non-Gemfile.lock files, instead of raising an exception

[postmodern]

Describe the problem as clearly as you can

```Bundler::LockfileParser#initialize will happily parse a non-Gemfile.lock` file, instead of raising an exception.

This causes downstream issues with bundler-audit when using the --gemfile-lock FILE option. While bundler-audit could check if the filename is Gemfile.lock or ends with the .lock extension, this is not sufficient as there is no way to validate the contents of the given file.

Did you try upgrading rubygems & bundler?

Yes. Currently running the latest stable version of Bundler.

Bundler::VERSION
# => "2.7.1"

Post steps to reproduce the problem

require 'bundler'
Bundler::LockfileParser.new(File.read('Gemfile')) # even README.md will be accepted

What happened instead?

#<Bundler::LockfileParser:0x00007f2d504899d0
 @dependencies={},
 @lockfile_path="Gemfile.lock",
 @most_specific_locked_platform=nil,
 @parse_method=nil,
 @platforms=[],
 @pos=
  #<Bundler::LockfileParser::Position:0x00007f2d4befc5e8 @column=1, @line=294>,
 @sources=[],
 @specs=[],
 @strict=false>

What did you expect?

An exception to be raised indicating that the given file is not a Gemfile.lock formatted file.