FIX: ignore global text list project files

[klauer]

Fix GlobalTextList loading bug

Projects with GlobalTextList entries in TwinCAT projects would previously raise:
RuntimeError: Unsupported xml type for TcSource: <Element TcPlcObject at 0x7f70fc315340>
For one, that exception message has been improved to include a bit more context:
UnsupportedSourceFileError: Unsupported xml type for TcSource: TcPlcObject/GlobalTextList

If loaded directly with the TcSource.from_filename utility method, the above will be raised.
If loaded as part of a solution/tsproj, the unsupported source file will just be ignored.

Test suite changes

• Update blark-twincat-root project to have some more unsupported file formats (and make sure blark doesn't fall over)
• Fix module-level pytest.skip error when users (or I) accidentally forget to clone recursively
• pcdshub repositories indexed in plc-summary (docs) are now all parsing correctly
  • One exception - a correctly-identified syntax error in some unused code: https://github.com/pcdshub/lcls-plc-tmo-vac/blob/6727eea7bbb48d255286257b1d024e23e9d42cad/plc-tmo-vac/PLC_TMO_VAC/POUs/FB_VRC_NO_FFO_VLS.TcPOU#L48 (trailing . of AND iq_stValve. THEN is syntactically incorrect)

[codecov-commenter]

Codecov Report

Merging #81 (d89aa43) into master (f7e0ecf) will increase coverage by 0.2%.
The diff coverage is 84.0%.

Additional details and impacted files

@@           Coverage Diff            @@
##           master     #81     +/-   ##
========================================
+ Coverage    79.6%   79.9%   +0.2%     
========================================
  Files          26      26             
  Lines        5491    5505     +14     
========================================
+ Hits         4374    4399     +25     
+ Misses       1117    1106     -11     

Files Changed	Coverage Δ
blark/tests/test_dependency_store.py	78.5% <ø> (ø)
blark/solution.py	79.5% <84.0%> (+0.6%)	⬆️

... and 1 file with indirect coverage changes

[klauer]

Tests all passing and this is looking good on my local parsing tests - merging