This commit introduces two functions (one for GNATCOLL.Projects, one for
GPR2) to build an analysis context object so that the unit provider,
preprocessing setup, configuration pragmas and the default charset are
automatically inferred from the project file.
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
