The call to checkdep_usetex() in __init__.py doesn't help for people
explicitly passing usetex=True. Moreover, it slows down import time
for those who do have text.usetex set to True in their rcfile by ~20ms
(when timing this, note that checkdep_ghostscript (which checkdep_usetex
calls) has a cache, which should be disabled for timing purposes.

Moreover, checkdep_usetex is overly broad, always requiring dvipng
(which is not required for pdf/ps output) and ghostscript (which is not
required for raster output).

Instead, move the check to texmanager.py, which will provide a more
meaningful error message to end users, speed up import time (20ms is
similar to the import time of pyparsing or urllib.request, for example)
and avoid the inaccuracies above.

xref #13284 (if the OP indeed is missing a tex installation).

PR Summary

PR Checklist

• Has Pytest style unit tests
• Code is Flake 8 compliant
• New features are documented, with examples if plot related
• Documentation is sphinx and numpydoc compliant
• Added an entry to doc/users/next_whats_new/ if major new feature (follow instructions in README.rst there)
• Documented in doc/api/api_changes.rst if API changed in a backward-incompatible way