Issue

Plugin names and plugin folders must exactly match (resolver_*, crypto_*)
Non-existing plugin variants are not checked
Default plugin collection is too small
Duplicate of all plugin names in different folder
Only one category per plugin
On some OS some plugins won't work (simpleini)

Constraints

Assumptions

Many plugins work without dependencies and variants, this should be simple

Considered Alternatives

have one more mapping that describes folder <-> plugins (bad coherence)
directly use README.md to also describe cmake deps (too limited in expression)
split up AddPlugin.cmake and CMakeLists.txt (does not work well with variants)
simply adding all directories in src/plugins and decide within the add_plugin if we should drop the plugin (see below in Argument)

Names for flag:

DEPENDENCIES -> might be used for add_plugin
ASSEMBLE_DEPS
FIND_DEPS -> not only finding happens
PROCESS_DEPS
HANDLE_DEPS
DEPS_MODE -> too generic, new terminology
CHECK_DEPS -> not only checking happens
DO_DEPS -> sounds funny
FIND_PACKAGES -> not only packages are subject to this phase
COLLECT -> needs NOT
FIND_PHASE -> needs NOT
DEPENDENCY -> ambiguous

Decision

Introduce a cmake process where all plugins are processed two times:

1.) collection phase (DEPENDENCY_PHASE is OFF), with:

ADDED_PLUGINS
REMOVED_PLUGINS
ADDED_DIRECTORIES
add_plugin, but without actually adding internally 2.) assemble dependency phase (DEPENDENCY_PHASE is ON, only considering ADDED_DIRECTORIES), with:
add_libraries, actually search for libraries on the system
add_plugin, with actually adding the plugins

Use following cmake variables for the phases:

COLLECTION_PHASE .. collect all add_plugins
DEPENDENCY_PHASE .. resolve all dependencies, do add_plugins again
ADDTESTING_PHASE .. (reserve for potential 3rd phase)

Argument

Solves all the issues without adding too much complexity. Can be extended to three phases if we need that for tests (they need knowledge about which plugins actually were added)

Maintaining additional mappings is very time-consuming. Simply adding all plugins directories is problematic. It would:

clutter the cmake output (especially in the case of errors)
introduce more variables into the CMakeCache which are irrelevant for the user
maybe even find libraries in wrong versions which are incompatible to what other plugins need

Implications

need to adopt all CMakeLists.txt to use find_package only in the second phase