using strace I am able to see that kded5 eventually looks at powerdevil in the right location, but still does not load it:

[~] # strace -p $(pidof kded5) -s 200 -f  2>&1 | grep power
[pid  1277] recvmsg(8, {msg_name(0)=NULL, msg_iov(1)=[{"l\1\0\1\17\0\0\0001\0\0\0n\0\0\0\1\1o\0\5\0\0\0/kded\0\0\0\6\1s\0\r\0\0\0org.kde.kded5\0\0\0\2\1s\0\r\0\0\0org.kde.kded5\0\0\0\3\1s\0\n\
0\0\0loadModule\0\0\0\0\0\0\10\1g\0\1s\0\0\7\1s\0\5\0\0\0:1.79\0\0\0\n\0\0\0powerdevil\0", 2048}], msg_controllen=0, msg_flags=MSG_CMSG_CLOEXEC}, MSG_CMSG_CLOEXEC) = 143
[pid  1277] stat(".//kf5/kded/powerdevil", 0x7ffcd719b7f0) = -1 ENOENT (No such file or directory)
[pid  1277] stat(".//kf5/kded/powerdevil.so", 0x7ffcd719b7f0) = -1 ENOENT (No such file or directory)
[pid  1277] stat(".//kf5/kded/libpowerdevil", 0x7ffcd719b7f0) = -1 ENOENT (No such file or directory)
[pid  1277] stat(".//kf5/kded/libpowerdevil.so", 0x7ffcd719b7f0) = -1 ENOENT (No such file or directory)
[pid  1277] stat("/usr/lib/x86_64-linux-gnu/qt5/plugins/kf5/kded/powerdevil", 0x7ffcd719b7f0) = -1 ENOENT (No such file or directory)
[pid  1277] stat("/usr/lib/x86_64-linux-gnu/qt5/plugins/kf5/kded/powerdevil.so", 0x7ffcd719b7f0) = -1 ENOENT (No such file or directory)
[pid  1277] stat("/usr/lib/x86_64-linux-gnu/qt5/plugins/kf5/kded/libpowerdevil", 0x7ffcd719b7f0) = -1 ENOENT (No such file or directory)
[pid  1277] stat("/usr/lib/x86_64-linux-gnu/qt5/plugins/kf5/kded/libpowerdevil.so", 0x7ffcd719b7f0) = -1 ENOENT (No such file or directory)
[pid  1277] stat("/usr/bin/kf5/kded/powerdevil", 0x7ffcd719b7f0) = -1 ENOENT (No such file or directory)
[pid  1277] stat("/usr/bin/kf5/kded/powerdevil.so", 0x7ffcd719b7f0) = -1 ENOENT (No such file or directory)
[pid  1277] stat("/usr/bin/kf5/kded/libpowerdevil", 0x7ffcd719b7f0) = -1 ENOENT (No such file or directory)
[pid  1277] stat("/usr/bin/kf5/kded/libpowerdevil.so", 0x7ffcd719b7f0) = -1 ENOENT (No such file or directory)
[pid  1277] write(2, "kf5.kded: kded module \"powerdevil\" still uses .desktop files (\"kded/powerdevil.desktop\"). Please port it to JSON metadata.\n", 123) = 123
[pid  1277] stat(".//powerdevil", 0x7ffcd719b890) = -1 ENOENT (No such file or directory)
[pid  1277] stat(".//powerdevil.so", 0x7ffcd719b890) = -1 ENOENT (No such file or directory)
[pid  1277] stat(".//libpowerdevil", 0x7ffcd719b890) = -1 ENOENT (No such file or directory)
[pid  1277] stat(".//libpowerdevil.so", 0x7ffcd719b890) = -1 ENOENT (No such file or directory)
[pid  1277] stat("/usr/lib/x86_64-linux-gnu/qt5/plugins/powerdevil", 0x7ffcd719b890) = -1 ENOENT (No such file or directory)
[pid  1277] stat("/usr/lib/x86_64-linux-gnu/qt5/plugins/powerdevil.so", 0x7ffcd719b890) = -1 ENOENT (No such file or directory)
[pid  1277] stat("/usr/lib/x86_64-linux-gnu/qt5/plugins/libpowerdevil", 0x7ffcd719b890) = -1 ENOENT (No such file or directory)
[pid  1277] stat("/usr/lib/x86_64-linux-gnu/qt5/plugins/libpowerdevil.so", 0x7ffcd719b890) = -1 ENOENT (No such file or directory)
[pid  1277] stat("/usr/bin/powerdevil", 0x7ffcd719b890) = -1 ENOENT (No such file or directory)
[pid  1277] stat("/usr/bin/powerdevil.so", 0x7ffcd719b890) = -1 ENOENT (No such file or directory)
[pid  1277] stat("/usr/bin/libpowerdevil", 0x7ffcd719b890) = -1 ENOENT (No such file or directory)
[pid  1277] stat("/usr/bin/libpowerdevil.so", 0x7ffcd719b890) = -1 ENOENT (No such file or directory)
[pid  1277] stat(".//kded_powerdevil", 0x7ffcd719b890) = -1 ENOENT (No such file or directory)
[pid  1277] stat(".//kded_powerdevil.so", 0x7ffcd719b890) = -1 ENOENT (No such file or directory)
[pid  1277] stat(".//libkded_powerdevil", 0x7ffcd719b890) = -1 ENOENT (No such file or directory)
[pid  1277] stat(".//libkded_powerdevil.so", 0x7ffcd719b890) = -1 ENOENT (No such file or directory)
[pid  1277] stat("/usr/lib/x86_64-linux-gnu/qt5/plugins/kded_powerdevil", 0x7ffcd719b890) = -1 ENOENT (No such file or directory)
[pid  1277] stat("/usr/lib/x86_64-linux-gnu/qt5/plugins/kded_powerdevil.so", {st_mode=S_IFREG|0644, st_size=254800, ...}) = 0
[pid  1277] open("/usr/lib/x86_64-linux-gnu/qt5/plugins/kded_powerdevil.so", O_RDONLY|O_CLOEXEC) = 31
[pid  1277] open("/usr/lib/x86_64-linux-gnu/qt5/plugins/kded_powerdevil.so", O_RDONLY|O_CLOEXEC) = 31
[pid  1277] open("/usr/lib/x86_64-linux-gnu/libpowerdevilcore.so.2", O_RDONLY|O_CLOEXEC) = 31
[pid  1277] stat("/usr/lib/x86_64-linux-gnu/qt5/plugins/kded_powerdevil.so", {st_mode=S_IFREG|0644, st_size=254800, ...}) = 0
[pid  1277] write(2, "kf5.kded: Could not load kded module \"powerdevil\":\"The shared library was not found.\" (library path was:\"powerdevil\")\n", 118) = 118