General: Added helper getters to modules manager

[iLLiCiTiT]

Brief description

Added helper methods to ModulesManager to access modules by name.

Description

Added dictionary like access to module by their names and special method to get module by name which also checks if is module enabled.

Testing notes:

New methods on created object of ModulesManager in different cases (e.g. in Tray > Admin > Console).

from openpype.modules import ModulesManager

manager = ModulesManager()
print(manager["project_manager"])
print(manager.get("project_manager"))
print(manager.get_enabled_module("project_manager"))

[kalisp]

Both new ones work as expected.