Catch2 compatibility means the API tests are written in: the macros, such
as TEST_CASE and REQUIRE, and the Catch:: namespace. It does not mean
Catch2's build names or header paths. find_package(Catch2) found Catch3,
and a project using both frameworks met two packages, two sets of targets,
two Catch.cmake modules and two catch2/ header trees with the same names.

Build names:
- CMake: project and package Catch3, targets Catch3::Catch3 and
  Catch3::Catch3WithMain, libraries Catch3 and Catch3Main, options
  CATCH3_*, lib/cmake/Catch3 with Catch3Config, and pkg-config catch3 and
  catch3-with-main. The version is Catch3's, 0.3.0, compatible within a
  minor version.
- The CMake scripts are Catch3.cmake (catch3_discover_tests),
  Catch3AddTests.cmake and Catch3ShardTests.cmake
  (catch3_add_sharded_tests); the helpers they define at ctest time have a
  catch3_ prefix. ParseAndAddCatchTests.cmake, deprecated upstream, is gone.
- Bazel //:catch3 and //:catch3_main, Meson catch3 and catch3-with-main
  (with override_dependency), Xmake catch3 and catch3_main, Premake Catch3
  and Catch3WithMain. Visual Studio projects get GUIDs of their own.
- The Progmasoft::Catch3 target and package are gone: property testing
  and snapshots are part of Catch3::Catch3. Their sources now build with
  the library's warning flags, so property failures are built field by
  field (no missing initializers) and the default property name is a
  thread-local pointer to the name its scope owns (no global destructor).

Headers: the engine moved from src/catch2 to src/Progmasoft/Catch3 with
Progmasoft names, so <catch2/catch_test_macros.hpp> is
<Progmasoft/Catch3/TestMacros.hpp>, <catch2/matchers/catch_matchers_string.hpp>
is <Progmasoft/Catch3/Matchers/String.hpp>, and interface headers are named
after their interface, such as Interfaces/IConfig.hpp. Include guards
became #pragma once. The installed headers go to include/Progmasoft.
src/Progmasoft/Context.cpp is StrategyContext.cpp, so no two sources share
a file name.

tools/scripts/upstreamFiles.txt lists the files that come from Catch2.
The license check expects BSL-1.0 on them and Progmasoft's header on the
rest of src/Progmasoft; the convenience-header check, the amalgamation
script and the surrogate tests take the engine's headers from it. The
amalgamated files are regenerated. The header-guard workflow checks for
#pragma once instead of checkguard.

Documents, the README and the release notes describe the new names.

Catch3 is maintained by Progmasoft, but the repository still carried
Catch2's funding links, its maintainer list and GPG keys, a Code of Conduct
contact at a Catch2 maintainer's address, its lists of users, its release
process and release scripts, its AppVeyor configuration and its logos.

- Removed FUNDING.yml, MAINTAINERS.md, appveyor.yml with the AppVeyor and
  OpenCppCoverage helpers, Catch2's user lists, why-catch, release process
  and deprecations pages, its two logos and the major/minor/patch release
  scripts. releaseCommon.py keeps only what the amalgamation reads.
- The Code of Conduct sends reports to support@progmasoft.com.
- contributing.md and release-notes.md are written for Catch3; Catch2's
  release history is linked upstream.
- CMake, pkg-config and Doxygen name Catch3 and its repository; the
  amalgamated files say which Catch3 version and which Catch2 engine they
  come from.

This enables us to generate pkg-config with proper paths if the
installation prefix is decided at install time, e.g. with
```
cmake --install build-dir --prefix /path/to/somewhere
```

It also means that we can use CMake's generator expression to get
the _real_ final name of the libraries, e.g. including debug postfix.

Closes #2979

This adds prefix and exec_prefix fields

This describes the reality better, as it also links in the rest
of Catch2.

The on-disk name of the static library remains just `Catch2Main`,
as that is what it is -- single main function -- and on-disk artifacts
cannot describe link dependencies.

Also fixes an installation issue, where some installation steps
would not properly pick up `CMAKE_INSTALL_PREFIX` when set.

This also goes for pkg-config installed by our CMake installation.

This includes

* Updating CMake version on Travis
* Adding a `Catch2` subfolder to the `single_include/` folder to
provide this include path both _inside_ the repository, and _outside_.
* Updated examples to build with the new paths
* Other general CMake cleanup