Allow to use AnalysisLoader dynamically with AnalysisHost

[Xanewok]

Currently the AnalysisHost<L: AnalysisLoader> can use the provided generic loader internally, via Vec<rls_data::Analysis> or from a single directory.

Per rust-lang/rls#988 (comment) it'd be good to be able to also reload the data using the AnalysisLoader dynamically (to not require manual data deserialization, when these are not necessarily in the same directory).