This reduces bootstrap cost for Windows customers, resolving the issue initially submitted as #12502 .

The `toolsrc` tree was extracted to https://github.com/microsoft/vcpkg-tool. `bootstrap.sh` was changed to download the right source tarball, extract, and build it. This was chosen over the previous attempt, a submodule, over concerns of accidentally destroying people's local modifications.

Co-authored-by: Robert Schumacher <roschuma@microsoft.com>

utilities.cmake: Disable warnings that are firing on Azure Pipelines CI machines due to different Clang version.

expected.h: Fix a bug I introduced in https://github.com/microsoft/vcpkg/pull/15638/ because I was under the impression expected worked like optional (in that value_or_exit for it should be treated as a program bug)

build.cpp: Add (void)s to silence warnings.

binarycaching.cpp: Repair assumption that the current directory is C: which isn't true on the Hosted Azure Pipelines agents.

others: Make unit tests respect %VCPKG_ROOT%, as necessary in the vcpkg_tool repo. Note that this required splitting vcpkgcmdarguments::ImbueFromEnvironment into the once-only process modifying part and the just imbue from environment part.

* [vcpkg] fix checking out git registry ports

* fix the new issues in builtinregistryentry

* fix tests

* fix tests boogaloo

* [vcpkg] Fix issue where baseline is requested for overlay during version resolution

* split BuiltinRegistryEntry into two types

Co-authored-by: Robert Schumacher <ras0219@outlook.com>

* [vcpkg] Miscellaneous internal improvements extracted from #15424

* [vcpkg] CR comments

* [armadillo] Use vcpkg_from_git() to workaround gitlab missing archive

Co-authored-by: Robert Schumacher <roschuma@microsoft.com>
Co-authored-by: Billy Robert O'Neal III <bion@microsoft.com>

In build https://dev.azure.com/vcpkg/public/_build/results?buildId=48398

we are getting output like:

```
Error: Building package v-hacd:x64-windows-static failed with: BUILD_FAILED
Elapsed time for package v-hacd:x64-windows-static: 4.156 s
Starting package 1396/1464: v8:x64-windows-static
Building package v8[core]:x64-windows-static...
-- Using msys root at D:/downloads/tools/msys2/969c0913b9df89e1
-- Fetching https://chromium.googlesource.com/v8/v8.git...
CMake Error at scripts/cmake/vcpkg_execute_required_process.cmake:108 (message):
    Command failed: C:/agent/externals/git/cmd/git.exe fetch https://chromium.googlesource.com/v8/v8.git 7565e93eb72cea4268028fc20186d415c22b1cff --depth 1 -n
    Working Directory: D:/downloads/git-tmp
    Error code: 128
    See logs for more information:
      D:\buildtrees\v8\git-fetch-x64-windows-static-err.log

Call Stack (most recent call first):
  scripts/cmake/vcpkg_from_git.cmake:91 (vcpkg_execute_required_process)
  ports/v8/portfile.cmake:71 (vcpkg_from_git)
  scripts/ports.cmake:128 (include)

Could not copy into file: D:\packages\v8_x64-windows-static\share\v8\vcpkg_abi_info.txt
```

but aren't printing the reason for the copy failure. Added printing of that reason.

* Fix warning on clang version 10.0.0-4ubuntu1

The warning was

```shell
../src/vcpkg/commands.porthistory.cpp:55:14: error: unused function 'is_date' [-Werror,-Wunused-function]
```

* Add environment variable VCPKG_USE_NUGET_CACHE

As the name suggests, this environment variable allow tu use Nuget
cache for Nuget binary caching sources.

* Document NuGet's Cache environment variable

* [vcpkg] Fix regression in error messages with registries/versioning

* [vcpkg] Clean up redundant re-parsing with try_load_overlay_port

* [vcpkg] Deduplicate PathsPortfileProvider

* [vcpkg] Restore uses of Checks::exit_maybe_upgrade after merge