Rename symbols pipelines names and refine descriptions

[pombredanne]

I suggest that we rename the pipelines for consistency:

• collect_symbols -> collect_symbols_ctags
  Collect source symbols with Ctags.

• collect_pygments_symbols -> collect_symbols_pygments
  Collect source symbols, string literals and comments with Pygments.

• collect_tree_sitter_symbols -> collect_symbols_tree_sitter
  Collect source symbols and string literals with Tree-sitter.

• collect_source_strings -> collect_strings_gettext
  Collect source string literals with xgettext.