Reconstruct globs when input and base are specs

When the base directory is an asset spec and an input file is also
an asset spec that contains a glob then the static view machinery
will be called with the glob.

In the past, this was not a problem because the `static_url` call
would always fail on the spec. Now that an asset spec in the
`webassets.base_dir` setting is honored with `webassets.static_view`
URLs containing a glob are returned instead of failing and falling
back to resolution using the filepath that was expanded during the
search for the source file.
With static_view add the externals separately

When `webassets.static_view` is used and the base directory is an asset
specification the `resolve_source_to_url` method of the resolver will
receive an absolute path to the pulled file and the original asset spec
for the input but has no access to the original base directory setting
in order to construct a new specification to pass to `static_url`. By
adding an additional static view for the externals using the absolute
path of the external directory these paths will resolve using the
Pyramid `static_url` machinery.
Clean up static view asset spec tests

These tests were somewhat confusing. They referred to base_url a lot
but actually meant to be setting the base_dir. Additionally, some
tests that seemed to pass should have been failing because they were
referencing non-existent files.

- Add the `static:some` temp files
- Change base_url parameterization to base_dir

This change causes a test to fail, which reveals an edge case in how
output URLs are handled. Relative outputs with a base directory as an
asset specification will not be resolved with static_url.
Resolve relative outputs with asset spec base dir

When the output is relative and the base directory is a static asset
the static view configuration will not be able to resolve the absolute
output path into a URL. To resolve it requires knowing the original
base directory setting, if it was an asset spec, so that the the call
to `static_url` can be passed an asset spec of the output.