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

Isabelle2016-1: syntax: use semantic markup instead of "header"

camkes: Port to Isabelle 2014.

The only major change is that "embed" is now a constant in HOL, removing
it from the set of valid names for free variables.

Have renamed uses of "embed" to "embed_data"; a better name could
probably be chosen by someone more familiar with the code.

Import release snapshot.