This commit introduces a new class that represents a loaded project
(GPRProject), created from the usual project loading arguments (project
file, scenario variables, target and RTS info). This new class provides
two methods: create_unit_provider(project=None) to create a unit
provider wrapping a project and source_files(mode) to query the list of
source files in a project.
It also removes the now obsolete SourceFiles class (recently added), but
preserves the UnitProvider.for_project method to avoid breaking existing
user code (been there for several years).
Closes#932
Move most of "ada/*" to the root directory (this makes sense, as this
repository has been dedicated to Libadalang for years), and rename
"ada/language" to "ada".
TN: T914-010
