Remove use of Foundation appending and add last FoundationEssentials imports

[madsodgaard]

I missed this one, but we were also calling appending from Foundation. Just changed to a normal string concatenation.

Also, this adds the last remaining import FoundationEssentials. OpenAPIRuntime now only links against FoundationEssentials 🎉

| LDD output:
| 	linux-vdso.so.1 (0x0000ffff883a5000)
| 	libswiftCore.so => /usr/lib/swift/linux/libswiftCore.so (0x0000ffff87a00000)
| 	libswift_Concurrency.so => /usr/lib/swift/linux/libswift_Concurrency.so (0x0000ffff88160000)
| 	libswift_StringProcessing.so => /usr/lib/swift/linux/libswift_StringProcessing.so (0x0000ffff880a0000)
| 	libswift_RegexParser.so => /usr/lib/swift/linux/libswift_RegexParser.so (0x0000ffff878e0000)
| 	libswiftSynchronization.so => /usr/lib/swift/linux/libswiftSynchronization.so (0x0000ffff878b0000)
| 	libswiftGlibc.so => /usr/lib/swift/linux/libswiftGlibc.so (0x0000ffff87880000)
| 	libFoundationEssentials.so => /usr/lib/swift/linux/libFoundationEssentials.so (0x0000ffff87200000)
| 	libm.so.6 => /lib/aarch64-linux-gnu/libm.so.6 (0x0000ffff87150000)
| 	libc.so.6 => /lib/aarch64-linux-gnu/libc.so.6 (0x0000ffff86f90000)
| 	/lib/ld-linux-aarch64.so.1 (0x0000ffff88368000)
| 	libstdc++.so.6 => /lib/aarch64-linux-gnu/libstdc++.so.6 (0x0000ffff86c00000)
| 	libgcc_s.so.1 => /lib/aarch64-linux-gnu/libgcc_s.so.1 (0x0000ffff87840000)
| 	libdispatch.so => /usr/lib/swift/linux/libdispatch.so (0x0000ffff86f10000)
| 	libswift_Builtin_float.so => /usr/lib/swift/linux/libswift_Builtin_float.so (0x0000ffff86ee0000)
| 	libBlocksRuntime.so => /usr/lib/swift/linux/libBlocksRuntime.so (0x0000ffff86eb0000)
| Success: Binary is not linked against libFoundation.so - linkage test passed.

[czechboy0]

Thanks, @madsodgaard 🎉