Deprecate Package.getInternalSubPackage, simplify DepedencyResolver

[Geod24]

The PackageManager will always eagerly load subpackages, whether they are path based or embedded (see addPackages). So calling getInternalSubPackage if getSubPackage returned null will never yield a non-Nullable object. In short, we can simplify that code greatly, and as Package.getInternalSubPackage does not handle all use case and require using the PackageManager anyway, just deprecate it and direct people to the right function.

[github-actions[bot]]

✅ PR OK, no changes in deprecations or warnings

Total deprecations: 0

Total warnings: 0

Build statistics:

 statistics (-before, +after)
-executable size=5286232 bin/dub
+executable size=5282136 bin/dub
 rough build time=61s

Full build output

``` DUB version 1.35.1, built on Jan 6 2024 LDC - the LLVM D compiler (1.36.0): based on DMD v2.106.1 and LLVM 17.0.6 built with LDC - the LLVM D compiler (1.36.0) Default target: x86_64-unknown-linux-gnu Host CPU: znver3 http://dlang.org - http://wiki.dlang.org/LDC Registered Targets: aarch64 - AArch64 (little endian) aarch64_32 - AArch64 (little endian ILP32) aarch64_be - AArch64 (big endian) amdgcn - AMD GCN GPUs arm - ARM arm64 - ARM64 (little endian) arm64_32 - ARM64 (little endian ILP32) armeb - ARM (big endian) avr - Atmel AVR Microcontroller bpf - BPF (host endian) bpfeb - BPF (big endian) bpfel - BPF (little endian) hexagon - Hexagon lanai - Lanai loongarch32 - 32-bit LoongArch loongarch64 - 64-bit LoongArch mips - MIPS (32-bit big endian) mips64 - MIPS (64-bit big endian) mips64el - MIPS (64-bit little endian) mipsel - MIPS (32-bit little endian) msp430 - MSP430 [experimental] nvptx - NVIDIA PTX 32-bit nvptx64 - NVIDIA PTX 64-bit ppc32 - PowerPC 32 ppc32le - PowerPC 32 LE ppc64 - PowerPC 64 ppc64le - PowerPC 64 LE r600 - AMD GPUs HD2XXX-HD6XXX riscv32 - 32-bit RISC-V riscv64 - 64-bit RISC-V sparc - Sparc sparcel - Sparc LE sparcv9 - Sparc V9 spirv32 - SPIR-V 32-bit spirv64 - SPIR-V 64-bit systemz - SystemZ thumb - Thumb thumbeb - Thumb (big endian) ve - VE wasm32 - WebAssembly 32-bit wasm64 - WebAssembly 64-bit x86 - 32-bit X86: Pentium-Pro and above x86-64 - 64-bit X86: EM64T and AMD64 xcore - XCore Upgrading project in /home/runner/work/dub/dub/ Starting Performing "release" build using /opt/hostedtoolcache/dc/ldc2-1.36.0/x64/ldc2-1.36.0-linux-x86_64/bin/ldc2 for x86_64. Building dub 1.36.0-beta.1+commit.48.gd17375a5: building configuration [application] Linking dub STAT:statistics (-before, +after) STAT:executable size=5282136 bin/dub STAT:rough build time=61s ```

[Geod24]

@thewilsonator : This one too ? ;)