[BUGFIX] Add "suggests" as valid dependency type

Currently extensions with "suggests" contraint in ext_emconf.php
fail to install in Extension Manager with exception
"suggestwas not a valid dependency type.".

Example (taken from EXT:saltedpasswords ext_emconf.php):
    'constraints' => array(
        [...]
        'suggests' => array(
            'rsaauth' => ''
        )
    ),

NOTE: This patch does not implement the missing handling
for suggested extensions.

Fixes: #42594
Releases: 6.0

Change-Id: I326de368508c5c9a350d7b7e84b07f3bc1582437
Reviewed-on: http://review.typo3.org/16367
Reviewed-by: Francois Suter
Tested-by: Francois Suter
Reviewed-by: Oliver Hader
Reviewed-by: Helmut Hummel
Tested-by: Helmut Hummel