builtins: skip addition of builtins if the library is added previously

[umarcor]

As discussed in #767, this PR modifies the functions for adding the built OSVVM and JSON-for-VHDL. Currently, a try...except block is used:

vunit/vunit/builtins.py

Lines 139 to 142 in 7879504

	try:
	library = self._vunit_obj.library(library_name)
	except KeyError:
	library = self._vunit_obj.add_library(library_name)

That approach has two main caveats. On the one hand, the try statement is not case sensitive. Hence, if a library named OSVVM exists, it does not get it when asking for osvvm. That falls back to the exception, which crashes because adding a library is case insensitive. If no crash is produced, additional sources are added to the existing library.

In this PR, _add_library_if_not_exist is added, which lowercases library names to check if it exists. Furthermore, if the library exists already, adding builtins is skipped, instead of mixing the content.

I didn't find any function in the API for retrieving the names of the libraries declared already. Hence, I used [library.lower() for library in self._vunit_obj._project._libraries]. However, we might want to add it in the project class.

[LarsAsplund]

How can we make sure that we will remember to update this message when OSVVM is bumped?

[umarcor]

I'm not sure... By navigating the codebase through the GitHub web, you don't get any explicit indication that OSVVM is bumped to a tag. It just points to the commit: https://github.com/OSVVM/OSVVM/tree/71faba779aebe79d3a48ce0abc0a9adc272f99ae.

We might use a test for checking out the history of OSVVM, using git for comparing the commit hash with the tags and find an exact match. Then, in the builtins.py, have the version moved to a variable which the check can import for comparison. However, I'm not sure that complexity is worth. Particularly, whether that should be part of any "regular" test.

We might just add a subsection to http://vunit.github.io/contributing.html about "How to bump a submodule", explaining that we want to bump to tagged commits only (in the case of OSVVM), and that whoever does it should remember to update the string.

When dependabot or some other similar solution supports bumping submodules to tags only, we might actually need an automated procedure.

BTW, the same applies to JSON-for-VHDL below.

[LarsAsplund]

I think the instruction is good enough for now. If we find out we keep missing to update the string we can come up with another solution.

[umarcor]

This PR now adds a subsection to the docs about bumping submodules.