Provide namespaced CMake targets

[dg0yt]

Description

Projects (e.g. msh3) that use CMake targets from dependencies (here: msquic) should use namespaced target names from those dependencies. This enables CMake to recognize the intention to use a target, not a library name. CMake can report a missing target definition early (at generation time), instead of creating linker instruction which may fail at build time.

msquic already added namespace msquic:: to the exported CMake config. However, this should be complemented with msquic:: ALIAS targets inside the build, for reverse dependencies which integrate msquic via add_subdirectory. This PR adds the missing targets.

In addition the PR shortens the exported name of msquic_platform to msquic::platform.

The msquic::base_link alias is added for use in msh3. (This nicely illustrate how the namespace clarifies the origin of base_link.)

Testing

Covered by src/platform/unittest/external/CMakeLists.txt.

The msquic/msh3 integration with namespaced targets is tested in https://github.com/microsoft/vcpkg/pull/43018/files, with a DLL build of msquic passed to msh3 via CMake config.(Blocked.)

Documentation

No change due to lack of specific CMake usage documentation.
(vcpkg prints an generated hint which will need to be reduced to linking msquic::msquic.)

[dg0yt]

This PR should be good, but the integration in vcpkg cannot complete, nibanks/msh3#235 (comment).

[nibanks]

CI is failing

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 84.96%. Comparing base (9610803) to head (4c7ff57).
Report is 5 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4737      +/-   ##
==========================================
- Coverage   85.71%   84.96%   -0.76%     
==========================================
  Files          56       56              
  Lines       17378    17378              
==========================================
- Hits        14896    14765     -131     
- Misses       2482     2613     +131     

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