cue/load: syntax issues in imported packages are overly repetitive and verbose

[mvdan]

! exec cue export

-- cue.mod/module.cue --
module: "github.com/username/reponame"
language: version: "v0.11.0"
-- input.cue --
package p

import "github.com/username/reponame/deeply/nested/imported"

out: imported.#Schema & {_in: "foo"}
-- deeply/nested/imported/imported.cue --
package imported

#Schema: {
	_in: string | *"default"
	out: _in
// Missing a closing curly brace.

The above results in a rather long error message:

> ! exec cue export
[stderr]
github.com/username/reponame@v0: import failed: cannot find package "github.com/username/reponame/deeply/nested/imported": cannot get imports: cannot read "deeply/nested/imported/imported.cue": expected '}', found 'EOF':
    ./input.cue:3:8
[exit status 1]

I see a few issues:

• "cannot find package X" is wrong and confusing. It was able to locate and start loading that package.
• "import failed" and "cannot get imports" on their own are unhelpful and redundant.
• deeply/nested/imported appears twice, which adds verbosity.
• "cannot read F" is also wrong; the issue was with parsing CUE syntax, not reading the file.

[mvdan]

Here is an example of the kind of error I would find a lot nicer:

> ! exec cue export
[stderr]
github.com/username/reponame/deeply/nested/imported: cannot parse "imported.cue": expected '}', found 'EOF':
    ./deeply/nested/imported/imported.cue:7:1
    ./input.cue:3:8
[exit status 1]

Note that I omitted the importer package from the error message, as I think it's less relevant. Also note that I add one more position to the error, which allows the message to be shorter as well.