all: remove theory import path references

In Isabelle2020, when isabelle jedit is started without a session
context, e.g. `isabelle jedit -l ASpec`, theory imports with path
references cause the isabelle process to hang.

Since sessions now declare directories, Isabelle can find those files
without path reference and we therefore remove all such path references
from import statements. With this, `jedit` and `build` should work with
and without explicit session context as before.

Signed-off-by: Gerwin Klein <gerwin.klein@data61.csiro.au>

licenses: convert license tags to SPDX

global: isabelle update_cartouches

sep_algebra: cleanup

- fixed legacy warnings
- fixed almost all other warnings as well
- clarified some monoid lemmas that had misleading mixed notation
- improved layout style compliance (seriously, people..)
- comment typo fixes etc

Import release snapshot.