install: make unit_file_get_list aware of UNIT_FILE_INDIRECT

install: make unit_file_get_list aware of UNIT_FILE_INDIRECT

Commit aedd401 introduced new unit file state, UNIT_FILE_INDIRECT. Unit file is
said to have indirect state if it contains [Install] section which has only
Also= directive. Thus, if enable of such unit file is requested then some other
unit file gets enabled.

Whether or not unit file is in indirect state can be determined by calling
unit_file_can_install. Function unit_file_get_list populates list of unit files
present in given lookup location. So far it did call unit_file_can_install in a
way that would prevent finding out about unit files in indirect state. Such unit
file would be incorrectly marked as static.

Fixes following assertion in test-install,

Assertion 'p->state == s' failed at src/test/test-install.c:59, function main(). Aborting.
[1]    26868 abort (core dumped)  ./test-install