Fix #21631 by improving error messages

[ras0219-msft]

Fixes microsoft/vcpkg#21631

Fundamentally, this bad experience was driven by trying to use the generic argument parser for two different commands based on hidden runtime information. I've chosen to resolve this issue by accepting the superset of behavior and provide more specific error messages when the superset does not apply to this case.

Before:

/workspaces/manifest $ vcpkg install zlib
Error: 'install' requires 0 arguments, but 1 were provided.
Example:
  vcpkg install --triplet x64-windows

Options:
  --dry-run                       Do not actually build or install
  --head                          Install the libraries on the command line using the latest
                                  upstream sources
...

After:

/workspaces/manifest $ vcpkg install zlib
Error: In manifest mode, `vcpkg install` does not support individual package arguments.
To install additional packages, edit vcpkg.json and then run `vcpkg install` without any package arguments.
See https://github.com/Microsoft/vcpkg/tree/master/docs/users/manifest.md for more information.
Using manifest file at /workspaces/manifest/vcpkg.json.

Example:
  vcpkg install --triplet x64-windows

Options:
  --dry-run                       Do not actually build or install
  --head                          Install the libraries on the command line using the latest
                                  upstream sources (classic mode)
...

[BillyONeal]

Hmmm this is an improvement but it leaves the help block very confused :(. I understand truly fixing that is going to require a more extensive overhaul though :(

[ras0219-msft]

I tried to minimize the confusion of the help block by adding a note for the "exclusive" options (head, editable, no-default-features, feature)