Add switch to force classic mode.

[BillyONeal]

A customer wanted to run vcpkg in a place where they could not control that a vcpkg.json was in a directory above, and requested a way to force classic mode. One used to be able to do this with the --no-manifest feature flag, but that was removed some years ago.

This adds a new switch, --classic, which effectively skips looking for a manifest.

Related: microsoft/vcpkg-docs#423

[walbourn]

Just curious why you didn't bring back the --no-manifest option? Did it do something else?

[BillyONeal]

Just curious why you didn't bring back the --no-manifest option? Did it do something else?

1. I believe it was named no-manifests with the extra S. (

   vcpkg-tool/src/vcpkg/vcpkgcmdarguments.cpp

   Line 267 in 471f4ef

   {FeatureFlagManifests, manifest_mode},

   ) I also believe it is better to spell options in a way where the user is declaring what they want, rather than absence of what they don't want.
2. I think (?) it may have had other effects than just selecting which mode we were in. Admittedly I didn't do much digging to find all the exact effects.
3. I really didn't want to use the feature flag mechanism to power something unrelated to feature flags.
4. I believe the actual number of users who expected no-manifests to do something is ~zero because it was several years in between removing the feature flag and anyone reporting this issue, so backwards compatibility isn't really a concern.
5. Backwards compatibility actually might have been a concern to reuse no-manifests since the product currently accepts it (but it has no effect), so making it do something else may subtly break anyone who ever passed it.