Flysystem v2 support

[mynameisbogdan]

Q	A
Branch?	2.0
Bug fix?	no
New feature?	yes
BC breaks?	no
Deprecations?	no
Tests pass?	yes
Fixed tickets	#1308
License	MIT
Doc PR

Adding support for Flysystem v2 as drop-in replacement in FlysystemLoaderFactory and FlysystemResolverFactory.

[coveralls]

Coverage increased (+0.6%) to 84.146% when pulling 98ce75a on mynameisbogdan:2.x into 7a9740c on liip:2.x.

[mynameisbogdan]

@lsmith77 I updated this PR.

I fixed some mismatches in the tests from the previous PR and put back the check on ExtensionGuesserInterface & MimeTypesInterface on the loader to maintain BC with Symfony pre-4.3.

[lsmith77]

thx .. also thx for removing phpcs dependency.

you re-added support for pre 4.3 versions because you need this yourself?

[mynameisbogdan]

No, I tried to maintain BC by keeping the same logic as the loader for v1. But now looking at composer.json, it makes sense to remove it since symfony/mime is mandatory even for symfony v3.

I pushed an update.

Thank you.

[fbourigault]

Is it possible to not introduce a flysystem2 configuration value and choose the proper implementation by detecting if V1 or V2 is installed?

[mynameisbogdan]

@fbourigault well that's case now. Since the loader and resolver must have an unique service id, I'm handling that in FlysystemLoaderFactory (eg liip_imagine.cache.resolver.prototype.flysystem2 / liip_imagine.binary.loader.prototype.flysystem2).

But the configuration value remained the same flysystem, so no BC breaks.

[lsmith77]

awesome thx.

[mynameisbogdan]

Thank you as well.

[lsmith77]

ah damn .. hit merge and then I remembered that we have failed to update the UPGRADING.md since a while but would like to get back to doing this .. could you add a new section with config instructions for users upgrading to flysystem2?

[mynameisbogdan]

@lsmith77 Well, there are no configuration changes. One can simply run composer require -W liip/imagine-bundle:2.x-dev league/flysystem:^2.0 and that's it.

Should I make a new PR with this information?

[lsmith77]

yes please

[mynameisbogdan]

Done in #1359