MonoAOTCompiler:
- Compiles assemblies to .bc files.
- Hashes for the .bc files are stored in a json cache file.
- And uses .depfile generated by mono-aot-cross, to figure out the
  if any of the dependencies have changed
- Writes out the actual .bc file only if the assembly, or it's
  dependencies changed

EmccCompile.cs: Support a `%(Dependencies)` metadata on the source
files, to compile the files only when needed.

otherwise xunit just shows a cryptic:
```
    Wasm.Build.Tests.RebuildTests.NoOpRebuild [STARTING]
    Wasm.Build.Tests.RebuildTests.NoOpRebuild [FAIL]
      System.NotSupportedException : Specified method is not supported.
```

This reverts commit 1d031c0.

This reverts commit b008130.

… file on disk doesn't exist

`--depfile` isn't supported on aot config used by android, and fails
with:

```
* Assertion at /__w/1/s/src/mono/mono/mini/aot-compiler.c:14216, condition `acfg->aot_opts.llvm_only && acfg->aot_opts.asm_only && acfg->aot_opts.llvm_outfile' not met
```

Instead, use hashes of the .bc.tmp files generated, with the existing
.bc files.

The earlier implementation assumed that there would be only one output
file. But in some cases (eg. android), there are more than one, like
`.s`, `.dll-llvm.o`.

.. used.