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

lib: improve wp tracing

When tracing wp can now print the instantiated version of the rules being used.
It also says which set each used rule is from.

lib: improve message printing for crunch and wp, and refactor common printing functions

lib: wp cleanup and parser improvements

The main visible change is from wp_trace', 'wp_once' and 'wp_once_trace' to
'wp (trace)', 'wp (once)' and 'wp (once, trace)'. The option for printing a
warning for unused supplied wp rules has also been removed.

global: isabelle update_cartouches

lib: Datatype_Schematic and WPFix.

Add two new tactics/methods which can fix common painful problems with
schematic variables.

Method datatype_schem improves unification outcomes, by making judicious use of
selectors like fst/snd/the/hd to bring variables into scope, and also using a
wrapper to avoid singleton constants like True being captured needlessly by
unification.

Method wpfix uses strengthen machinery to instantiate rogue postcondition
schematics to True and to split precondition schematics that are shared across
different sites.

wp: separate wp_pre method, used in wp and wpc

apply_debug: add "wp" breakpoint

Lift low-level wp tactic into context_tactic to support
breakpoints modifying context (i.e. wp set)

wp: more constrained behaviour

- restrict to one goal (no fall-through)
- apply a wp_pre rule if there is no schematic in the goal
- apply trivial cleanup rules at the end

lib: start disentangling spaghetti word dependencies