cmTarget: Don't allow relative paths in INTERFACE_SOURCES

Follow the pattern of checks that are made for INTERFACE_INCLUDE_DIRECTORIES.
Existence is already checked by cmSourceFile::GetFullPath.  Add a check
to disallow relative paths in source directories. Otherwise code such as

 target_sources(lib1 INTERFACE foo.cpp)

would fail if consumed by a target in a different directory.

Unlike the INTERFACE_INCLUDE_DIRECTORIES behavior, we don't care whether
the entry comes from an IMPORTED target or not.  In the include directories
case, the directory for a non-imported target might not exist yet but
might be created.  In the sources case, a file which does not yet
exist in the filesystem must be explicitly marked with the GENERATED
property.

Adjust existing tests and add a new test for the error.
Export: Disallow export of targets with INTERFACE_SOURCES

This can be allowed in the next release, but it needs to have some
features present and tested such as

* Ensuring that relative paths do not appear in the generated property.
* Ensuring that paths to the source or build directories do not appear.
* Generating a check in the file for CMake 3.1 or later so that the
    resulting property will be consumed.
* Ensuring that any referenced targets are part of an export set and
    generating a check for them.
* INSTALL_INTERFACE and BUILD_INTERFACE content.

All of these checks are already done for INTERFACE_INCLUDE_DIRECTORIES,
but it is too late to add them for INTERFACE_SOURCES for CMake 3.1.
As the checks introduce some new error conditions, it is better to
disallow exporting fully for this case and introduce proper error
conditions later instead of policies.
Help: Document the export limitation of INTERFACE_SOURCES.
Merge topic 'target-sources-error-conditions' into next

f38d80c6 Help: Document the export limitation of INTERFACE_SOURCES.
84f2cba5 Export: Disallow export of targets with INTERFACE_SOURCES
495ae31f cmTarget: Don't allow relative paths in INTERFACE_SOURCES