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>

lib: add ML_goal command

Sometimes we want to prove a fact, but the fact is painful or
error-prone to type out manually. In these cases, we'd like to construct
the goal fact using ML and then immediately enter a proof block.

Previously, we could achieve something like this through careful use of
`Thm.trivial` and `schematic_goal`, but this would clutter up the ML
namespace and wouln't handle meta conjuncts (`&&&`). The new `ML_goal`
command addresses both of these issues.

Signed-off-by: Edward Pierzchalski <ed.pierzchalski@data61.csiro.au>