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: use `@{term_pat}` in FP_Eval; refactor term_pat testsuite

lib/FP_Eval: add license headers

lib/FP_Eval: move let_weak_cong' to main tool; improve docs

lib: initial version of FP_Eval tool

FP_Eval is an Isabelle/ML tool for functional program rewriting.
It has similarities with the Isabelle simplifier, but is simpler and
more scalable for performing computations in the logic.

See FP_Eval_Tests for basic tests and examples.