pygmt.show_versions: Remove _get_gdal_version and create a dictionary for dependency versions

[seisman]

Description of proposed changes

1. Remove _get_gdal_version and create a dictionary for dependency versions The private function was added in Let pygmt.show_versions show GDAL version #3364.
2. Move imports out of the functions
3. Check gs_version=None in _check_ghostscript_version May fail to find ghostscript as shown in Module 'psconvert' failed with status code 79: psconvert [ERROR]: Cannot execute Ghostscript (gswin64c). #3366.

>>> import pygmt

>>> pygmt.show_versions()
PyGMT information:
  version: v0.12.1.dev78+g9c13eb04b.d20240709
System information:
  python: 3.12.3 | packaged by conda-forge | (main, Apr 15 2024, 18:38:13) [GCC 12.3.0]
  executable: /home/seisman/opt/miniforge/envs/pygmt/bin/python3.12
  machine: Linux-6.9.9-200.fc40.x86_64-x86_64-with-glibc2.39
Dependency information:
  numpy: 2.0.0
  pandas: 2.2.2
  xarray: 2024.6.0
  netCDF4: 1.6.5
  packaging: 24.1
  contextily: 1.6.0
  geopandas: 1.0.0
  IPython: 8.25.0
  rioxarray: 0.15.6
  gdal: 3.9.0
  ghostscript: 10.03.1
GMT library information:
  version: 6.5.0
  padding: 2
  share dir: /home/seisman/opt/miniforge/envs/pygmt/share/gmt
  plugin dir: /home/seisman/opt/miniforge/envs/pygmt/lib/gmt/plugins
  library path: /home/seisman/opt/miniforge/envs/pygmt/lib/libgmt.so
  cores: 80
  grid layout: rows
  image layout:
  binary version: 6.5.0

[weiji14]

If ghostscript version cannot be detected, should there be a warning raised here?

[seisman]

See https://github.com/GenericMappingTools/pygmt/pull/3376/files#r1706342731