Support absolute Windows paths with forward slashes in common::FindFile

[traversaro]

🦟 Bug fix

Summary

While working on gazebosim/gz-sim#1574, some tests were failing with error:

    Start 43: UNIT_ign_TEST

43: Test command: C:\src\ign-gazebo\build\bin\UNIT_ign_TEST.exe "--gtest_output=xml:C:/src/ign-gazebo/build/test_results/UNIT_ign_TEST.xml"
43: Environment variables:
43:  IGN_CONFIG_PATH=C:/src/ign-gazebo/build/test/conf
43:  IGN_GAZEBO_SYSTEM_PLUGIN_PATH=C:/src/ign-gazebo/build/bin
43: Test timeout computed to be: 240
43: Running main() from C:\src\ign-gazebo\test\gtest\src\gtest_main.cc
43: [==========] Running 3 tests from 1 test suite.
43: [----------] Global test environment set-up.
43: [----------] 3 tests from CmdLine
43: [ RUN      ] CmdLine.Server
43: Running command [ C:/Users/STraversaro/AppData/Local/mambaforge/envs/igngaz/Library/bin/ign gazebo -s  -r -v 4 --iterations 5 C:/src/ign-gazebo/test/worlds/plugins.sdf]
43: C:\src\ign-gazebo\src\ign_TEST.cc(73): error: Expected: (output.find("iteration " + std::to_string(i))) != (std::string::npos), actual: 18446744073709551615 vs 18446744073709551615
43: [Msg] Ignition Gazebo Server v6.10.0
43: [Err] [D:\bld\libignition-common4_1656525324646\work\src\SystemPaths.cc:379] Unable to find file with URI [C:///src/ign-gazebo/test/worlds/plugins.sdf]
43: [Err] [D:\bld\libignition-common4_1656525324646\work\src\SystemPaths.cc:469] Could not resolve file [C:/src/ign-gazebo/test/worlds/plugins.sdf]
43: [Err] [C:\src\ign-gazebo\src\Server.cc:109] Failed to find world [C:/src/ign-gazebo/test/worlds/plugins.sdf]

Apparently what was happening is that the C:/src/ign-gazebo/test/worlds/plugins.sdf path was interpreted as an URI, resulting in the strange error on C:///src/ign-gazebo/test/worlds/plugins.sdf . While technical C:/ could be an URI with the one-letter scheme C, in practice I am not aware of any one letter URI scheme, and on the other hand many Windows programs supports both absolute paths with backslash and forward slashes, see for example Python:

(igngaz) C:\src\ign-gazebo\build>type hello.py
print("Hello world")

(igngaz) C:\src\ign-gazebo\build>python C:\src\ign-gazebo\build\hello.py
Hello world

(igngaz) C:\src\ign-gazebo\build>python C:/src/ign-gazebo/build/hello.py
Hello world

To support this use case, this PR modifies the common::FindFile function to first check if a file that starts with C:/ or similar is an existing absolute file in the system. If such a file does not exist, then the search procedure falls back to consider it an URI.

Checklist

• Signed all commits for DCO
• Added tests
• Updated documentation (as needed)
• Updated migration guide (as needed)
• Consider updating Python bindings (if the library has them)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
• While waiting for a review on your PR, please help review another open pull request to support the maintainers

Note to maintainers: Remember to use Squash-Merge and edit the commit message to match the pull request summary while retaining Signed-off-by messages.

[codecov]

Codecov Report

Merging #389 (d7141c4) into ign-common4 (edb3509) will increase coverage by 0.00%.
The diff coverage is 90.90%.

@@             Coverage Diff              @@
##           ign-common4     #389   +/-   ##
============================================
  Coverage        77.03%   77.03%           
============================================
  Files               76       76           
  Lines            10735    10736    +1     
============================================
+ Hits              8270     8271    +1     
  Misses            2465     2465           

Impacted Files	Coverage Δ
src/SystemPaths.cc	86.83% <90.90%> (+0.05%)	⬆️

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